Modernize Remarkup CSS

Summary: Uses standard spacing and colors in Remarkup. Also removed 'remarkup dark' since it doesn't exist anymore (Pholio). Left font sizes in em's for spacing.

Test Plan: Tested a few dozen Diviner pages, my wiki pages, and a few tasks.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8439