Improve Legalpad Preamble UI

Summary: Darken up the Preamble UI and provide more dramatic spacing. Fixes T9866

Test Plan: Review a preamble.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T9866

Differential Revision: https://secure.phabricator.com/D14579