To set your favourite font to be on GitHub you need:

1. Refined GitHub extension
2. Go to options of this extension. For Chrome: click on 3 dots on installed extension on Extensions list and select Options
3. Paste following CSS into Custom CSS text-area

pre,
code,
.blob-code,
.blob-code-marker,
.input-monospace,
.markdown-body code {
  font-family: MonoLisa;
}

1. You can change fonts/colours for other parts of GitHub using Custom CSS option