R1:8bbcd896b806

Add styling for new Remarkup highlighter

Summary: Adds some basic style to new !!Remarkup Highlighter!! Ref T5560

Test Plan: Wait for next diff.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T5560

Differential Revision: https://secure.phabricator.com/D14383