Minor touchup to Paste Embed UI

Summary: Consolidate colors / spacing.

Test Plan:
Test embedding a paste, a list of pastes, and a PasteView for new colors, space.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11896