Add support for GitHub's new Colorblind themes

[kamekku14]

Echoing from #166 and #176, there are two new Colorblind themes, which swaps the reds and greens with oranges and blues, making it work for colorblind users. To activate it, head to the user menu at the top navbar, go to Feature preview and enable Colorblind Themes.

I would like to see if this browser extension fits very well for this theme.