Lots of little minor CSS tweaks

Summary: Lots of little details, fix workboard bg colors, darken up global backgrounds just a hair, add more "widgety" look to dashboard panels, remove underline on anchors on mobile. Also Fixes T12210

Test Plan: Use lots of pages on mobile and desktop.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12210

Differential Revision: https://secure.phabricator.com/D17315