Weekend header labels in month view should have darker backgrounds instead of dark actual weekend days.

Summary: Closes T8186, Weekend header labels in month view should have darker backgrounds instead of dark actual weekend days.

Test Plan: open month view, observe weekend labels having a darker background.

Reviewers: chad, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8186

Differential Revision: https://secure.phabricator.com/D12860