Punch up grey button border grey 10%

Summary: Visually these are hard to see on blue backgrounds, adds a touch more contrast. Fixes T12604

Test Plan: View as pager and dialog

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12604

Differential Revision: https://secure.phabricator.com/D17755