Distraction-free dark Google Docs style. Requires the Stylish add-on for Firefox. Add a new style and copy the CSS below.

dark-google-docs-firefox.css

@namespace url(http://www.w3.org/1999/xhtml);

@-moz-document url-prefix("https://docs.google.com/document/") {
  #docs-editor {
    background-color: #000000 !important;
  }

  .kix-page {
    background-color: #000000 !important;
    border-top: 0px !important;
  }

  .kix-paginateddocumentplugin {
    border: 0px !important;
  }
    
  .docs-ui-unprintable:last-child {
    background-color: #000000 !important;
  }
    
  .kix-cursor-caret {
    border-color: #FFFFFF !important;
  }

  .kix-page, .kix-page * {
    color: #ffffff !important;
  }
}