How to add custom CSS to any website
  • Set toolkit.legacyUserProfileCustomizations.stylesheets in about:config to true

  • Go to about:support to find out where your profile directory is located, e.g. ~/.mozilla/firefox/n2sdfb6i.default/

  • Inside that directory create a directory called chrome

  • Inside chrome create a file called userContent.css with the following contents:

    /* Make sure that PAGEDOWN always scrolls a full screen down */
    body {
      margin-bottom: 91vh !important;
    }
    
    /* Applies only to example.com */
    @-moz-document domain(example.com) {
      div {
        color: white !important;
        background-color: black !important;
      }
    }
    
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