GitHub Fixed Header for better accessibility.

stylish.css

/**
1. Install the Stylish(https://chrome.google.com/webstore/detail/stylish/fjnbnpbmkenffdnngjfgmeleoegfcffe?hl=en) extension for Chrome.
2. Open up extension options and paste the whole CSS mentioned below.
3. Specify the domain name to be `github.com`.
4. Add a title and save.
*/

@media (min-height: 850px) {
  .header {
      padding-top: 10px;
      padding-bottom: 10px;
      background-color: #f5f5f5;
      border-bottom: 1px solid #e5e5e5;
      position: fixed;
      top: 0;
      width: 100%;
      min-width: 1020px;
      z-index: 99999;
  }

  div[role="main"] {
      padding-top: 49px !important;
  }
}

Added a media query so it only takes effect on windows taller than 850px.
Also added min-width on .header to fix its position on smaller width windows.

That's sleek!