GitHub Gists default CSS selectors and base styles.

GitHub Gist Basic Styles

.gist {
  color: #000;

  .render-container {
    .render-viewer-error,
    .render-viewer-fatal,
    .octospinner {
      display: none;
    }
  }
  .gist-render iframe {
    width: 100%;
  }

  pre {
    font: inherit;
    line-height: inherit;
    white-space: pre;
  }

  .gist-file {
    border: 1px solid #ddd;
    border-bottom: 1px solid #ccc;
    border-radius: 3px;
    font-family: Consolas, "Liberation Mono", Courier, monospace;
    margin-bottom: 1em;

    &.gist-render {
      border: none;
    }

    .gist-meta {
      font: 12px "Helvetica Neue", Helvetica, arial, freesans, clean, sans-serif;
      text-shadow: 1px 1px rgba(255, 255, 255, 0.8);
      overflow: hidden;
      color: #999;
      padding: 10px;
      background-color: #e8e8e8;
      background-image: -moz-linear-gradient(#fafafa, #e8e8e8);
      background-image: -webkit-linear-gradient(#fafafa, #e8e8e8);
      background-image: linear-gradient(#fafafa, #e8e8e8);
      background-repeat: repeat-x;

      a {
        font-weight: bold;
        color: #666;
        text-decoration: none;
      }
    }

    .gist-data {
      overflow: auto;
      word-wrap: normal;
      background-color: #f8f8ff;
      border-bottom: 1px solid #ddd;
      font-size: 100%
    }

    &.scroll {
      .gist-data {
        position: absolute;
        top: 0;
        right: 0;
        bottom: 30px;
        left: 0;
        overflow: scroll;
      }

      .gist-meta {
        position: absolute;
        bottom: 0;
        right: 0;
        left: 0;
      }
    }

    .gist-data {
      .line-data {
        padding: .5em !important
      }

      .line-pre {
        font-family: Consolas, "Liberation Mono", Courier, monospace;
        background: transparent !important;
        border: none !important;
        margin: 0 !important;
        padding: 0 !important;
      }

      .gist-highlight {
        background: transparent !important;
      }

      .line-numbers {
        background-color: #ececec;
        color: #aaa;
        border-right: 1px solid #ddd;
        text-align: right;
        padding: .5em;

        .line-number {
          clear: right;
          display: block;
        }
      }

    }
  }
}

.gist-syntax {
  background: #fff;

  .err {
    color: #a61717;
    background-color: #e3d2d2;
  }

  .k,
  .o {
    color: #000;
    font-weight: bold;
  }

  .c,
  .cm,
  .c1 {
    color: #998;
    font-style: italic;
  }

  .cp,
  .cs {
    color: #999;
    font-weight: bold;
  }

  .cs {
    font-style: italic;
  }

  .gd {
    color: #000;
    background-color: #fdd;
    .x {
      color: #000;
      background-color: #faa;
    }
  }

  .ge {
    color: #000;
    font-style: italic;
  }

  .gr {
    color: #a00;
  }

  .gh {
    color: #999;
  }

  .gi {
    color: #000;
    background-color: #dfd;
    .x {
      color: #000;
      background-color: #afa;
    }
  }

  .go {
    color: #888;
  }

  .gp {
    color: #555;
  }

  .gs {
    font-weight: bold;
  }

  .gu {
    color: #aaa;
  }

  .gt {
    color: #a00;
  }

  .kc,
  .kd,
  .kp,
  .kr {
    color: #000;
    font-weight: bold;
  }

  .kt {
    color: #445588;
    font-weight: bold;
  }

  .s {
    color: #d14;
  }

  .nb {
    color: #0086b3;
  }

  .nc {
    color: #445588;
    font-weight: bold;
  }

  .ni {
    color: #800080;
  }

  .ne,
  .nf {
    color: #900;
    font-weight: bold;
  }

  .nn {
    color: #555;
  }

  .nt {
    color: #000080;
  }

  .ow {
    color: #000;
    font-weight: bold;
  }

  .w {
    color: #bbb;
  }

  .il,
  .m,
  .mf,
  .mh,
  .mi,
  .mo {
    color: #099;
  }

  .sb,
  .sc,
  .s2,
  .se,
  .sh,
  .si,
  .sx,
  .sb,
  .sb,
  .sb,
  .s1 {
    color: #d14
  }

  .sr {
    color: #009926
  }

  .ss {
    color: #990073
  }

  .bp {
    color: #999
  }

  .vc,
  .vg,
  .vi,
  .na,
  .nv,
  .no {
    color: #008080;
  }
}

I think the class is file