Any idea how to do this?

You need first find out how do you want to style the website. You can use the devtools menu for that F12. Then you need to create a userContent.css, and write your css there. I recommend writing your rules for given urls or url-prefixes like @-moz-document url-prefix(www.4chan.org){body{background: #111}} or @-moz-document url("www.4chan.org"){body{background: #111}} so it only has an effect on the websites you want to style.