Add dark color scheme to GH buttons

[redbrain]

If implemented, this pull request changes the bright white "star" and "fork" buttons to dark gray, matching the darkness of the rest of the webpage.

Note: You could also replace the octocat icons on each button with star and fork icons by adding the attributes data-icon="octicon-star" and data-icon="octicon-repo-forked" to the star and fork buttons respectively. However, I didn't want to change more than just the darkness in case it wasn't wanted. If you have write access you can edit this patch anyways.

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