Using Firefox/Stylish

Stylish is a Firefox extension that allows you to apply custom Cascading Style Sheets (CSS) style on a per-site basis.

Following privacy issues appearing from January 2017, Stylish was pulled from both the Chrome Web Store and Mozilla Add-ons in July 2018, and was automatically uninstalled for all existing users, as per Wikipedia. A version of Stylish that did not have these privacy issues seems to be Stylish 2.0.7[VERIFY].

Stylus is an extension that is a fork of Stylish 1.5.2.

Stylem is an extension that is a fork of Stylish 2.0.7, designed to work in Pale Moon, a Firefox fork. It also works in SeaMonkey 2.49.5, a Firefox-related suite.

Stylish lets you specify the domains to which the CSS shall apply like this: @-moz-document domain(wikipedia.org), domain(wikibooks.org) { /* Your CSS */ }

One use of Stylish is to remove elements of web sites, for example as follows:

@-moz-document domain(youtube.com) { }
 * 1) player-ads {display: none !important;}

Above, "#player-ads" is the selector, "display: none" ensures the removing of the element, and "!important" ensures the rule has a high priority. In some web browsers, you can find out about the selector for an element by choosing Inspect Element context menu, finding the right element, and then choosing Copy > Selector or the like.

See also Cascading Style Sheets Wikibook, including its Selectors chapter.