Firefox Readermode dark sidebar theme + hidden + show on hover + yellow selection + yellow link color

userContent.css

/*
   how to install:
   - visit about:support
   - open profile folder
   - create/edit chrome\userContent.css file
*/

@-moz-document url-prefix("about:reader") {
  /* fix toolbar */
  .toolbar {
    opacity: 0;
    background-color: #222 !important;
    border-right: 1px solid #555 !important;
  }
  .toolbar:hover {
    opacity: 1;
    background-color: #222 !important;
    border-right: 1px solid #555 !important;
  }
  .button {
    color: #cecece !important;
    background-color: #222 !important;
  }

  .toolbar .button {
    border-right: 1px solid #555 !important;
    border-bottom: 1px solid #555 !important;
  }

  /* fix selections */
  .moz-reader-content ::selection,
  ::selection {
    background: darkgoldenrod !important;
    color: #222 !important;
  }

  /* fix link colors */
  .moz-reader-content a:visited,
  a:visited {
    color: goldenrod !important;
  }
  .moz-reader-content a:link,
  a:link {
    color: gold !important;
  }
  .moz-reader-content a:hover,
  .moz-reader-content a:active,
  a:hover,
  a:active {
    color: khaki !important;
  }
}