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Altering Stylesheet with javascript? [duplicate]

Is it possible to make changes to a CSS rule-set dynamically (i.e. some JS which would change a CSS rule-set when the user clicks a widget)
This particular CSS rule-set is applied to lots of elements (via a class selector) on the page and I want to modify it when the user clicks the widget, so that all the elements having the class change.

You can, but it's rather cumbersome. The best reference on how to do it is the following article: Totally Pwn CSS with Javascript (web archive link).
I managed to get it to work with Firefox and IE - I couldn't in Chrome, though it appears that it supports the DOM methods.ricosrealm reports that it works in Chrome, too.

This is a modern version based on Totally Pwn CSS with Javascript. It's ES6 I hope don't mind.
function getCSSRule(ruleName) {
ruleName = ruleName.toLowerCase();
var result = null;
var find = Array.prototype.find;
find.call(document.styleSheets, styleSheet => {
result = find.call(styleSheet.cssRules, cssRule => {
return cssRule instanceof CSSStyleRule
&& cssRule.selectorText.toLowerCase() == ruleName;
});
return result != null;
});
return result;
}
This function returns a CSSStyleRule that you can use like this:
var header = getCSSRule('#header');
header.style.backgroundColor = 'red';
Also document.styleSheets list references of the CSSStylesSheets Objects. Other way to acces a specific sytleSheet in the page is by assigning an id to the style or link element in the html code, and get it in javascript using document.getElementById('my-style').sheet. This are some useful methods:
Major Browsers and IE9+ : insertRule(), deleteRule(), removeProperty().
Major Browsers, Firefox? and IE9+ : setProperty().
<stye id="my-style" ...
....
var myStyle = document.getElementById('my-style').sheet
myStyle.insertRule('#header { background: red; }', 0);
It is also possible to dynamically create a new style element to store dynamic created styles, I think should be way to avoid conflicts.

You can edit CLASS in document styleshets as follows
[...document.styleSheets[0].cssRules].find(x=> x.selectorText=='.box')
.style.background= 'red';
function edit() {
[...document.styleSheets[0].cssRules].find(x=> x.selectorText=='.box')
.style.background= 'red';
}
.box {
margin: 10px;
padding: 10px;
background: yellow;
}
<button onclick="edit()" >Click me</button>
<div class="box" >My box 1</div>
<div class="box" >My box 2</div>
<div class="box" >My box 3</div>

I tried the code via link from #alex-gyoshev comment, but it dosn't work
it fails on the CSS rules with Google fonts in Chrome
it fails on FireFox security checks
So I changed it slightly, but deleted delete functionality since it wasn't needed for me. Checked in IE 11, FireFox 32, Chrome 37 and Opera 26.
function getCSSRule(ruleName) { // Return requested style object
ruleName = ruleName.toLowerCase(); // Convert test string to lower case.
var styleSheet;
var i, ii;
var cssRule = false; // Initialize cssRule.
var cssRules;
if (document.styleSheets) { // If browser can play with stylesheets
for (i = 0; i < document.styleSheets.length; i++) { // For each stylesheet
styleSheet = document.styleSheets[i];
if (!styleSheet.href) {
if (styleSheet.cssRules) { // Browser uses cssRules?
cssRules = styleSheet.cssRules; // Yes --Mozilla Style
} else { // Browser usses rules?
cssRules = styleSheet.rules; // Yes IE style.
} // End IE check.
if (cssRules) {
for (ii = 0; ii < cssRules.length; ii++) {
cssRule = cssRules[ii];
if (cssRule) { // If we found a rule...
// console.log(cssRule);
if (cssRule.selectorText) {
console.log(cssRule.selectorText);
if (cssRule.selectorText.toLowerCase() == ruleName) { // match ruleName?
return cssRule; // return the style object.
}
}
}
}
}
}
}
}
return false; // we found NOTHING!
}

Depending on what you're trying to achieve, a better solution might be to change/add a class to a containing element (body would do!), and define classes accordingly.
.yourclass { color: black }
#wrapper.foo .yourclass { color: red }
#wrapper.bar .yourclass { color: blue }
then you can just use
document.getElementById('wrapper').className='foo';
(or your chosen js framework's wrapper for the same) to change everything with class yourclass inside whatever your wrapper element is.

The APIs for editing stylesheets with JS are, sadly, not consistent across browsers. The YUI Stylesheet Utility attempts to smooth over these differences so you could just use that. You could also look at the source code to figure out how it works if you don't want to use YUI itself.

give your style tag an id, like <style id="ssID">
if someonelse is making your styles for you
tell THAT person to give the style tag an id -
that way you can access it directly without
scrambling around wondering what its index is
// create a hash table
var cssHash = {};
// loop through and populate the hash table
for (let i in (r = ss0.sheet.rules)) {
// selectorText is the name of the rule - set the value equal to the rule
cssHash[r[i].selectorText] = r[i];
}
now you have a hash table for everything in the style sheet -
note that some values will be undefined, but not for
any of the things you care about
if you have, for instance, a class called #menuItem
and you want to change its color to black, do this
cssHash['#menuItem'].style.color = #000;
that line will set the color of the style of the rule
whose index was looked up in the hash table (cssHash)
by the name '#menuItem'
more importantly, you probably have several different
classes that you want to change all at once
kind of like when you switched majors in college
let's say you have four different classes
and you want to set all of their background colors
to the same value, that some user selected from an input
the color selector tag is <input id="bColor" type="color">
and the class rules you want to change are called
#menuItem .homeAddr span and #vacuum:hover
// create a listener for that color selector
bColor.addEventListener('input', function (e) {
// loop through a split list of the four class names
'#menuItem .homeAddr span #vacuum:hover'.split(' ').forEach(function (obj) {
// use the hash table to look up the index of each name
// and set the background color equal to the color input's value
cssHash[obj].style.backgroundColor = bColor.value;
});
}, false); // false added here for the sake of non-brevity

While setAttribute is nice, there is a standard way of doing this across most browsers:
htmlElement.className = 'someClass';
To do it over many elements, you will need a cross browser solution:
function getElementsByClassName( className, context, tagName ) {
context = context || document;
if ( typeof context.getElementsByClassName === 'function' )
return context.getElementsByClassName( className );
if ( typeof context.getElementsByTagName !== 'function' )
return [];
var elements = typeof tagName === 'string' ? context.getElementsByTagName( tagName ) :
context.getElementsByTagName('*'),
ret = [];
for ( var i = 0, il = elements.length; i < il; i++ )
if ( elements[ i ].className.match( className ) )
ret.push( elements[ i ] );
return ret;
}
var elements = getElementsByClassName('someClass');
for ( var i = 0, il = elements.length; i < il; i++ )
elements[ i ].className = 'newClass';
You may want to replace the line:
if ( elements[ i ].className.match( className ) )
With some Regular Expression, but you will have to escape special characters in that case.

To check all stylesheets for the rule and set it:
Your rule:
.aaa: {
background-color: green
}
[...document.styleSheets].flatMap(s=>[...s.cssRules])
.find(i=>i.selectorText=='.aaa').style.backgroundColor = 'red';
Note that the css styles, when accessed through javascript, do not have dashes in them. In the example above, background-color becomes backgroundColor

successfully reverse the transition with js ,but why my another way doesn't work?

my demo jsfiddle
What I want is a reverse transition when I click the < li > again, but the commentted code didn`t work,and the code below works fine
let dbclickre = true;
function flipped() {
if (dbclickre) {
document.querySelector(".linkrec").setAttribute("Id", "flipped");
} else {
document.querySelector(".linkrec").removeAttribute("Id", "flipped")
}
dbclickre = !dbclickre;
}
below is the commentted code (I think when i firstly click the last < li > ,js will excute the if statement(and indeed it works fine),but when i click again , the else statement didn't excude(but i have set #flipped .reverse {background: whitesmoke} ) . why this happening???)
// const dbclickre = document.querySelector(".reverse");
// function flipped() {
// if (dbclickre.style.backgroundColor = 'white') {
// document.querySelector(".linkrec").setAttribute("Id", "flipped");
// } else {
// document.querySelector(".linkrec").removeAttribute("Id", "flipped")
// }
// }

Instead of relying on background color for checking the state of flip, you could check for existence of Id attribute. Here the the changed code:
const dbclickre = document.querySelector(".reverse");
function flipped() {
if ( document.querySelector(".linkrec").getAttribute("Id") == undefined ) {
document.querySelector(".linkrec").setAttribute("Id", "flipped");
} else {
document.querySelector(".linkrec").removeAttribute("Id", "flipped")
}
}
Edit:
Why element.style would not work?
From MDN Web Docs:
The style property is used to get as well as set the inline style of an element.
Hence, the style property would not work with embedded or external CSS.
Also, it may not be a good idea to use hard-coded colors as the condition, because changing colors in the respective CSS classes would completely break the functionality.

ng-mouseover ng-mouseout not woking

below is my code, I'm trying to make the content wrapped in div tag change the background color when the mouse curse over it, if the one of the content's variable starts with *. But it doesn't work...
// html
<style>
.normal{background-color: white}
.change{background-color: gainsboro}
</style>
<div ng-mouseover="checkAs(this)" ng-mouseout="this.className='normal'">
......
</div>
// js
$scope.checkAs = function(obj) {
var name = $scope.opportunity.name;
var asterisk = '*';
if(name.startsWith(asterisk)) {
obj.className='change';
} else {
obj.className='normal';
}
};

If you are determined to do this in angular, you would have to call a function through ng-mouseover and in that function, you would need a selector such as JQuery or Javascript's query selector, then modify the element as you see fit. You would have to do something like this (using JQuery):
$scope.checkAs = function() {
$("div").hover(function() {
$(this).prop('background-color','gainsboro');
}, function(){
$(this).prop('background-color','white');
});
};
But, as PSL suggested, the "this" in checkAs(this) won't be the DOM element. A CSS solution might be better:
div :hover{
background-color: gainsboro
}

Change :hover CSS properties with JavaScript

How can JavaScript change CSS :hover properties?
For example:
HTML
<table>
<tr>
<td>Hover 1</td>
<td>Hover 2</td>
</tr>
</table>
CSS
table td:hover {
background:#ff0000;
}
How can the td :hover properties be modified to, say, background:#00ff00, with JavaScript? I know I could access the style background property using JavaScript with:
document.getElementsByTagName("td").style.background="#00ff00";
But I don't know of a .style JavaScript equivalent for :hover.

Pseudo classes like :hover never refer to an element, but to any element that satisfies the conditions of the stylesheet rule. You need to edit the stylesheet rule, append a new rule, or add a new stylesheet that includes the new :hover rule.
var css = 'table td:hover{ background-color: #00ff00 }';
var style = document.createElement('style');
if (style.styleSheet) {
style.styleSheet.cssText = css;
} else {
style.appendChild(document.createTextNode(css));
}
document.getElementsByTagName('head')[0].appendChild(style);

You can't change or alter the actual :hover selector through Javascript. You can, however, use mouseenter to change the style, and revert back on mouseleave (thanks, #Bryan).

Pretty old question so I figured I'll add a more modern answer. Now that CSS variables are widely supported they can be used to achieve this without the need for JS events or !important.
Taking the OP's example:
<table>
<tr>
<td>Hover 1</td>
<td>Hover 2</td>
</tr>
</table>
We can now do this in the CSS:
table td:hover {
// fallback in case we need to support older/non-supported browsers (IE, Opera mini)
background: #ff0000;
background: var(--td-background-color);
}
And add the hover state using javascript like so:
const tds = document.querySelectorAll('td');
tds.forEach((td) => {
td.style.setProperty('--td-background-color', '#00ff00');
});
Here's a working example https://codepen.io/ybentz/pen/RwPoeqb

What you can do is change the class of your object and define two classes with different hover properties. For example:
.stategood_enabled:hover { background-color:green}
.stategood_enabled { background-color:black}
.stategood_disabled:hover { background-color:red}
.stategood_disabled { background-color:black}
And this I found on:
Change an element's class with JavaScript
function changeClass(object,oldClass,newClass)
{
// remove:
//object.className = object.className.replace( /(?:^|\s)oldClass(?!\S)/g , '' );
// replace:
var regExp = new RegExp('(?:^|\\s)' + oldClass + '(?!\\S)', 'g');
object.className = object.className.replace( regExp , newClass );
// add
//object.className += " "+newClass;
}
changeClass(myInput.submit,"stategood_disabled"," stategood_enabled");

Sorry to find this page 7 years too late, but here is a much simpler way to solve this problem (changing hover styles arbitrarily):
HTML:
<button id=Button>Button Title</button>
CSS:
.HoverClass1:hover {color: blue !important; background-color: green !important;}
.HoverClass2:hover {color: red !important; background-color: yellow !important;}
JavaScript:
var Button=document.getElementById('Button');
/* Clear all previous hover classes */
Button.classList.remove('HoverClass1','HoverClass2');
/* Set the desired hover class */
Button.classList.add('HoverClass1');

If it fits your purpose you can add the hover functionality without using css and using the onmouseover event in javascript
Here is a code snippet
<div id="mydiv">foo</div>
<script>
document.getElementById("mydiv").onmouseover = function()
{
this.style.backgroundColor = "blue";
}
</script>

You can use mouse events to control like hover.
For example, the following code is making visible when you hover that element.
var foo = document.getElementById("foo");
foo.addEventListener('mouseover',function(){
foo.style.display="block";
})
foo.addEventListener('mouseleave',function(){
foo.style.display="none";
})

I'd recommend to replace all :hover properties to :active when you detect that device supports touch. Just call this function when you do so as touch()
function touch() {
if ('ontouchstart' in document.documentElement) {
for (var sheetI = document.styleSheets.length - 1; sheetI >= 0; sheetI--) {
var sheet = document.styleSheets[sheetI];
if (sheet.cssRules) {
for (var ruleI = sheet.cssRules.length - 1; ruleI >= 0; ruleI--) {
var rule = sheet.cssRules[ruleI];
if (rule.selectorText) {
rule.selectorText = rule.selectorText.replace(':hover', ':active');
}
}
}
}
}
}

This is not actually adding the CSS to the cell, but gives the same effect. While providing the same result as others above, this version is a little more intuitive to me, but I'm a novice, so take it for what it's worth:
$(".hoverCell").bind('mouseover', function() {
var old_color = $(this).css("background-color");
$(this)[0].style.backgroundColor = '#ffff00';
$(".hoverCell").bind('mouseout', function () {
$(this)[0].style.backgroundColor = old_color;
});
});
This requires setting the Class for each of the cells you want to highlight to "hoverCell".

I had this need once and created a small library for, which maintains the CSS documents
https://github.com/terotests/css
With that you can state
css().bind("TD:hover", {
"background" : "00ff00"
});
It uses the techniques mentioned above and also tries to take care of the cross-browser issues. If there for some reason exists an old browser like IE9 it will limit the number of STYLE tags, because the older IE browser had this strange limit for number of STYLE tags available on the page.
Also, it limits the traffic to the tags by updating tags only periodically. There is also a limited support for creating animation classes.

Declare a global var:
var td
Then select your guiena pig <td> getting it by its id, if you want to change all of them then
window.onload = function () {
td = document.getElementsByTagName("td");
}
Make a function to be triggered and a loop to change all of your desired td's
function trigger() {
for(var x = 0; x < td.length; x++) {
td[x].className = "yournewclass";
}
}
Go to your CSS Sheet:
.yournewclass:hover { background-color: #00ff00; }
And that is it, with this you are able to to make all your <td> tags get a background-color: #00ff00; when hovered by changing its css propriety directly (switching between css classes).

For myself, I found the following option: from https://stackoverflow.com/a/70557483/18862444
const el = document.getElementById('elementId');
el.style.setProperty('--focusHeight', newFocusHeight);
el.style.setProperty('--focusWidth', newFocusWidth);
.my-class {
--focusHeight: 32px;
--focusWidth: 256px;
}
.my-class:focus {
height: var(--focusHeight);
width: var(--focusWidth);
}

You can make a CSS variable, and then change it in JS.
:root {
--variableName: (variableValue);
}
to change it in JS, I made these handy little functions:
var cssVarGet = function(name) {
return getComputedStyle(document.documentElement).getPropertyValue(name);
};
and
var cssVarSet = function(name, val) {
document.documentElement.style.setProperty(name, val);
};
You can make as many CSS variables as you want, and I haven't found any bugs in the functions;
After that, all you have to do is embed it in your CSS:
table td:hover {
background: var(--variableName);
}
And then bam, a solution that just requires some CSS and 2 JS functions!

Had some same problems, used addEventListener for events "mousenter", "mouseleave":
let DOMelement = document.querySelector('CSS selector for your HTML element');
// if you want to change e.g color:
let origColorStyle = DOMelement.style.color;
DOMelement.addEventListener("mouseenter", (event) => { event.target.style.color = "red" });
DOMelement.addEventListener("mouseleave", (event) => { event.target.style.color = origColorStyle })
Or something else for style when cursor is above the DOMelement.
DOMElement can be chosen by various ways.

I was researching about hover, to be able to implement them in the button label and make the hover effect
<button type="submit"
style=" background-color:cornflowerblue; padding:7px; border-radius:6px"
onmouseover="this.style.cssText ='background-color:#a8ff78; padding:7px; border-radius:6px;'"
onmouseout="this.style.cssText='background-color:cornflowerblue; padding:7px; border-radius:6px'"
#click="form1()">
Login
</button>

You can create a class in css
.hover:hover {
background: #ff0000;
}
and then add it dynamically
const columns = document.querySelectorAll('table td');
for (let i = 0; i < columns.length; i++) {
columns[i].classList.add('hover');
}
But your css and js files should be connected in index.html

const tds = document.querySelectorAll('td');
tds.forEach((td,index) => {
td.addEventListener("mouseover", ()=>hover(index))
td.addEventListener("mouseout", ()=>normal(index))
});
function hover(index){
tds[index].style.background="red";
}
function normal(index){
tds[index].style.background="yellow";
}
Try this code it will work fine .

If you use lightweight html ux lang, check here an example, write:
div root
.onmouseover = ev => {root.style.backgroundColor='red'}
.onmouseleave = ev => {root.style.backgroundColor='initial'}
The code above performes the css :hover metatag.

Changing a CSS rule-set from Javascript

Is it possible to make changes to a CSS rule-set dynamically (i.e. some JS which would change a CSS rule-set when the user clicks a widget)
This particular CSS rule-set is applied to lots of elements (via a class selector) on the page and I want to modify it when the user clicks the widget, so that all the elements having the class change.

You can, but it's rather cumbersome. The best reference on how to do it is the following article: Totally Pwn CSS with Javascript (web archive link).
I managed to get it to work with Firefox and IE - I couldn't in Chrome, though it appears that it supports the DOM methods.ricosrealm reports that it works in Chrome, too.

This is a modern version based on Totally Pwn CSS with Javascript. It's ES6 I hope don't mind.
function getCSSRule(ruleName) {
ruleName = ruleName.toLowerCase();
var result = null;
var find = Array.prototype.find;
find.call(document.styleSheets, styleSheet => {
result = find.call(styleSheet.cssRules, cssRule => {
return cssRule instanceof CSSStyleRule
&& cssRule.selectorText.toLowerCase() == ruleName;
});
return result != null;
});
return result;
}
This function returns a CSSStyleRule that you can use like this:
var header = getCSSRule('#header');
header.style.backgroundColor = 'red';
Also document.styleSheets list references of the CSSStylesSheets Objects. Other way to acces a specific sytleSheet in the page is by assigning an id to the style or link element in the html code, and get it in javascript using document.getElementById('my-style').sheet. This are some useful methods:
Major Browsers and IE9+ : insertRule(), deleteRule(), removeProperty().
Major Browsers, Firefox? and IE9+ : setProperty().
<stye id="my-style" ...
....
var myStyle = document.getElementById('my-style').sheet
myStyle.insertRule('#header { background: red; }', 0);
It is also possible to dynamically create a new style element to store dynamic created styles, I think should be way to avoid conflicts.

You can edit CLASS in document styleshets as follows
[...document.styleSheets[0].cssRules].find(x=> x.selectorText=='.box')
.style.background= 'red';
function edit() {
[...document.styleSheets[0].cssRules].find(x=> x.selectorText=='.box')
.style.background= 'red';
}
.box {
margin: 10px;
padding: 10px;
background: yellow;
}
<button onclick="edit()" >Click me</button>
<div class="box" >My box 1</div>
<div class="box" >My box 2</div>
<div class="box" >My box 3</div>

I tried the code via link from #alex-gyoshev comment, but it dosn't work
it fails on the CSS rules with Google fonts in Chrome
it fails on FireFox security checks
So I changed it slightly, but deleted delete functionality since it wasn't needed for me. Checked in IE 11, FireFox 32, Chrome 37 and Opera 26.
function getCSSRule(ruleName) { // Return requested style object
ruleName = ruleName.toLowerCase(); // Convert test string to lower case.
var styleSheet;
var i, ii;
var cssRule = false; // Initialize cssRule.
var cssRules;
if (document.styleSheets) { // If browser can play with stylesheets
for (i = 0; i < document.styleSheets.length; i++) { // For each stylesheet
styleSheet = document.styleSheets[i];
if (!styleSheet.href) {
if (styleSheet.cssRules) { // Browser uses cssRules?
cssRules = styleSheet.cssRules; // Yes --Mozilla Style
} else { // Browser usses rules?
cssRules = styleSheet.rules; // Yes IE style.
} // End IE check.
if (cssRules) {
for (ii = 0; ii < cssRules.length; ii++) {
cssRule = cssRules[ii];
if (cssRule) { // If we found a rule...
// console.log(cssRule);
if (cssRule.selectorText) {
console.log(cssRule.selectorText);
if (cssRule.selectorText.toLowerCase() == ruleName) { // match ruleName?
return cssRule; // return the style object.
}
}
}
}
}
}
}
}
return false; // we found NOTHING!
}

Depending on what you're trying to achieve, a better solution might be to change/add a class to a containing element (body would do!), and define classes accordingly.
.yourclass { color: black }
#wrapper.foo .yourclass { color: red }
#wrapper.bar .yourclass { color: blue }
then you can just use
document.getElementById('wrapper').className='foo';
(or your chosen js framework's wrapper for the same) to change everything with class yourclass inside whatever your wrapper element is.

The APIs for editing stylesheets with JS are, sadly, not consistent across browsers. The YUI Stylesheet Utility attempts to smooth over these differences so you could just use that. You could also look at the source code to figure out how it works if you don't want to use YUI itself.

give your style tag an id, like <style id="ssID">
if someonelse is making your styles for you
tell THAT person to give the style tag an id -
that way you can access it directly without
scrambling around wondering what its index is
// create a hash table
var cssHash = {};
// loop through and populate the hash table
for (let i in (r = ss0.sheet.rules)) {
// selectorText is the name of the rule - set the value equal to the rule
cssHash[r[i].selectorText] = r[i];
}
now you have a hash table for everything in the style sheet -
note that some values will be undefined, but not for
any of the things you care about
if you have, for instance, a class called #menuItem
and you want to change its color to black, do this
cssHash['#menuItem'].style.color = #000;
that line will set the color of the style of the rule
whose index was looked up in the hash table (cssHash)
by the name '#menuItem'
more importantly, you probably have several different
classes that you want to change all at once
kind of like when you switched majors in college
let's say you have four different classes
and you want to set all of their background colors
to the same value, that some user selected from an input
the color selector tag is <input id="bColor" type="color">
and the class rules you want to change are called
#menuItem .homeAddr span and #vacuum:hover
// create a listener for that color selector
bColor.addEventListener('input', function (e) {
// loop through a split list of the four class names
'#menuItem .homeAddr span #vacuum:hover'.split(' ').forEach(function (obj) {
// use the hash table to look up the index of each name
// and set the background color equal to the color input's value
cssHash[obj].style.backgroundColor = bColor.value;
});
}, false); // false added here for the sake of non-brevity

While setAttribute is nice, there is a standard way of doing this across most browsers:
htmlElement.className = 'someClass';
To do it over many elements, you will need a cross browser solution:
function getElementsByClassName( className, context, tagName ) {
context = context || document;
if ( typeof context.getElementsByClassName === 'function' )
return context.getElementsByClassName( className );
if ( typeof context.getElementsByTagName !== 'function' )
return [];
var elements = typeof tagName === 'string' ? context.getElementsByTagName( tagName ) :
context.getElementsByTagName('*'),
ret = [];
for ( var i = 0, il = elements.length; i < il; i++ )
if ( elements[ i ].className.match( className ) )
ret.push( elements[ i ] );
return ret;
}
var elements = getElementsByClassName('someClass');
for ( var i = 0, il = elements.length; i < il; i++ )
elements[ i ].className = 'newClass';
You may want to replace the line:
if ( elements[ i ].className.match( className ) )
With some Regular Expression, but you will have to escape special characters in that case.

To check all stylesheets for the rule and set it:
Your rule:
.aaa: {
background-color: green
}
[...document.styleSheets].flatMap(s=>[...s.cssRules])
.find(i=>i.selectorText=='.aaa').style.backgroundColor = 'red';
Note that the css styles, when accessed through javascript, do not have dashes in them. In the example above, background-color becomes backgroundColor