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.