How do you add CSS with Javascript?

The simple-and-direct approach is to create and add a new style node to the document.

// Your CSS as text
var styles = `
    .qwebirc-qui .ircwindow div { 
        font-family: Georgia,Cambria,"Times New Roman",Times,serif;
        margin: 26px auto 0 auto;
        max-width: 650px;
    }
    .qwebirc-qui .lines {
        font-size: 18px;
        line-height: 1.58;
        letter-spacing: -.004em;
    }
    
    .qwebirc-qui .nicklist a {
        margin: 6px;
    }
`

var styleSheet = document.createElement("style")
styleSheet.innerText = styles
document.head.appendChild(styleSheet)

You can also do this using DOM Level 2 CSS interfaces (MDN):

var sheet = window.document.styleSheets[0];
sheet.insertRule('strong { color: red; }', sheet.cssRules.length);

...on all but (naturally) IE8 and prior, which uses its own marginally-different wording:

sheet.addRule('strong', 'color: red;', -1);

There is a theoretical advantage in this compared to the createElement-set-innerHTML method, in that you don't have to worry about putting special HTML characters in the innerHTML, but in practice style elements are CDATA in legacy HTML, and ‘<’ and ‘&’ are rarely used in stylesheets anyway.

You do need a stylesheet in place before you can started appending to it like this. That can be any existing active stylesheet: external, embedded or empty, it doesn't matter. If there isn't one, the only standard way to create it at the moment is with createElement.

Shortest One Liner

// One liner function:
const addCSS = css => document.head.appendChild(document.createElement("style")).innerHTML=css;

// Usage: 
addCSS("body{ background:red; }")

You can add classes or style attributes on an element by element basis.

For example:

<a name="myelement" onclick="this.style.color='#FF0';">text</a>

Where you could do this.style.background, this.style.font-size, etc. You can also apply a style using this same method ala

this.className='classname';

If you want to do this in a javascript function, you can use getElementByID rather than 'this'.

This easy example of add <style> in head of html

var sheet = document.createElement('style');
sheet.innerHTML = "table th{padding-bottom: 0 !important;padding-top: 0 !important;}\n"
+ "table ul {    margin-top: 0 !important;    margin-bottom: 0 !important;}\n"
+ "table td{padding-bottom: 0 !important;padding-top: 0 !important;}\n"
+ ".messages.error{display:none !important;}\n"
+ ".messages.status{display:none !important;} ";

document.body.appendChild(sheet); // append in body
document.head.appendChild(sheet); // append in head

Source Dynamic style - manipulating CSS with JavaScript

YUI just recently added a utility specifically for this. See stylesheet.js here.

if you know at least one <style> tag exist in page , use this function :

CSS=function(i){document.getElementsByTagName('style')[0].innerHTML+=i};

usage :

CSS("div{background:#00F}");

Here's a sample template to help you get started

Requires 0 libraries and uses only javascript to inject both HTML and CSS.

The function was borrowed from the user @Husky above

Useful if you want to run a tampermonkey script and wanted to add a toggle overlay on a website (e.g. a note app for instance)

// INJECTING THE HTML
document.querySelector('body').innerHTML += '<div id="injection">Hello World</div>';

// CSS INJECTION FUNCTION
//https://stackoverflow.com/questions/707565/how-do-you-add-css-with-javascript
function insertCss( code ) {
    var style = document.createElement('style');
    style.type = 'text/css';
    if (style.styleSheet) {
        // IE
        style.styleSheet.cssText = code;
    } else {
        // Other browsers
        style.innerHTML = code;
    }
    document.getElementsByTagName("head")[0].appendChild( style );
}

// INJECT THE CSS INTO FUNCTION
// Write the css as you normally would... but treat it as strings and concatenate for multilines
insertCss(
  "#injection {color :red; font-size: 30px;}" +
  "body {background-color: lightblue;}"
)

In modern browsers, you can use document.adoptedStyleSheets to add CSS.

const sheet = new CSSStyleSheet();
sheet.replace("strong { color: red; }");
document.adoptedStyleSheets = [...document.adoptedStyleSheets, sheet];

One advantage of this approach is that you do not have to wait for the <head> element to even become available, which may be a concern in browser extension code that runs very early.

I always forget how to add a class to an HTML element and this SO comes up early in Google, but no one has added the modern way of doing this so here goes.

To add a CSS style you can select the element and call .classList.add(<className>)

for example: document.querySelector("#main").classList.add("bg-primary");

You may also need to remove other class(es) which clash with the one you add. To do so: document.querySelector("#main").classList.remove("bg-secondary");

That's it. Run the sample and you'll see the setInterval() method add & remove the styles every 3 seconds.

let useSecondary = false;

setInterval(changeBgColor, 3000);

function changeBgColor(){
  if (useSecondary){
    document.querySelector("#main").classList.remove("bg-primary");
    document.querySelector("#main").classList.add("bg-secondary");
  }
  else{
    document.querySelector("#main").classList.remove("bg-secondary");
    document.querySelector("#main").classList.add("bg-primary");
  }
  useSecondary = !useSecondary;
}

* {
    transition: all 0.5s ease-in-out;
}

.bg-primary {
   background-color: green;
}

.bg-secondary{
   background-color: yellow;
}

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>JS Bin</title>
</head>
<body>
<div >
    <div id="main" >
        Example text has background color changed every 3 seconds by adding / removing CSS styles.
    </div>
</div>
</body>
</html>

use .css in Jquery like $('strong').css('background','red');

$('strong').css('background','red');

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<strong> Example
</strong>