Clean up remarkup table UI

Summary: Little more header spacing, more blu-ish inline with other elements.

Test Plan:
test a table before and after.

{F281094}

{F281095}

Reviewers: epriestley, btrahan

Reviewed By: btrahan

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11620