Migrate Project workboard background color to modular transactions

Summary: Removes now-unused method as well. Fixes T12673.

Test Plan: UI fiddling.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley

Maniphest Tasks: T12673

Differential Revision: https://secure.phabricator.com/D18014