Hello!

Here's a new theme that is based off 'github-dark' to match Github's current Dark Mode.

We had a brief discussion (#462) and I realize the "github-dark" theme will be updated when Primer is updated. This made me think it might be useful to create a separate theme, so that changes to Primer or GitHub's Dark Mode won't affect the theme.

I named it dark-mood, and I'm not attached to it, so if you want to rename go for it!

Please let me know if anything else is needed.

Thank you!

Here's a screenshots, from my local build: