Provide syntax highlighting css for oblivious phame posts

Summary: I have simply copied the existing css into the oblivious skin. I don't know if this is the right approach (code duplication), but considering this skin should be isolated (and will potentially differ) I think this makes sense.

Test Plan: Use a code block on a phame post.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin, chad

Differential Revision: https://secure.phabricator.com/D6313