[Redesign] Mobile menus / headers

Summary: Ref T8099, make the main menu and search menu lighter to match new UI.

Test Plan: set browser to smaller breakpoints, test mobile menus and search.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D12958