Adjust the shade of green in blame

Summary:
We are loading blame on background which confuses me a bit - I'm not sure if it's still loading or it's already loaded and the commit changed lots of lines (so I don't see the author) and it was a long time ago (so I don't see green).
Provide a visual feedback even for very old commits.

I want to point out that I really enjoy this kind of work.
Also, this diff is my masterpiece at least for today.

Test Plan: ?view=blame - verified that the gray changed to a decent shade of green even for very old commits.

Reviewers: epriestley, chad

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5292