added some more rules for dark remarkup environments

Summary:
Added rules for <tt> (` [inline code]) (did bright text on somewhat dark bg), NOTE: block and tables. These were all the ones I could think of and found that they required triage.

I did not touch minor things like links etc., which may require refinement. I try to stay away from design details.

Test Plan: {F35940}

Reviewers: chad, epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5345