userContent.css

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

@-moz-document url-prefix(https://www.google.co.jp/reader),
               url-prefix(https://www.google.com/reader) {

  #gb:not(:hover),
  #gb:not(:hover) > * {
    max-height: 5px !important;
  }

  #gb:not(:hover) .gbtc,
  #gb:not(:hover) #gbg {
    display: none !important;
  }

  #gb:not(:hover) ~ #top-bar {
    display: none;
  }

  #gb(:hover) {
    max-height: none !important;
  }

  #top-bar:hover,
  #gb:hover ~ #top-bar {
    display: block !important;
  }

}