[Redesign] Normalize colors on PHUIListView

Summary: Darker greys, less background colors, proper borders. Ref T8099

Test Plan: Tested a tab panel and Files application.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8099

Differential Revision: https://secure.phabricator.com/D12924