gitlabhq/app/assets/stylesheets/sections/themes.scss

.themes_opts {
  padding-left:20px;

  label {
    width:175px;
    margin-right:40px;

    .prev {
      @extend .thumbnail;
      height:30px;
      width:175px;
      margin-bottom:10px;

      &.classic {
        background: #31363e;
      }

      &.default {
        background: #f1f1f1;
      }

      &.modern {
        background: #567;
      }

      &.gray {
        background: #708090;
      }

      &.violet {
        background: #657;
      }
    }
  }
}

.code_highlight_opts {
  padding-left:20px;

  label {
    width:220px;
    margin-right:40px;

    .prev {
      @extend .thumbnail;
      height:151px;
      width:220px;
      margin-bottom:10px;
    }
  }
}