Use white for active state on mobile menus

Summary: Perviously we used {$sky} but we've since change the icons to .8 opacity on white, allowing use of white on select.

Test Plan: Click on the mobile menu, see icon turn white instead of blue.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12430