Add support for GitHub's new Dark High Contrast theme

[kamekku14]

Echoing from issue #166, there's now a new Dark High Contrast theme for GitHub—and it's exactly the same as GitHub's Dark theme, but with better contrast. To activate it, head to the user menu at the top navbar, go to Feature preview and enable Dark High Contrast.

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