Move code examples in remarkup to grey backgrounds
Summary: This should make code easier to read. Right now large blocks get a really bright treatment, and this should tone it down. Also upped the spacing a bit.
Test Plan:
Review Remarkup Examples from Diviner, also some test code blocks and counter examples.
{F1081606}
{F1081607}
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: Korvin
Differential Revision: https://secure.phabricator.com/D15141