R1:ebecbeed4237

Rebuild lightbox styles

Summary: Simpler, lighter background and UI for lightbox. Removes images, uses font icons, buttons.

Test Plan:
Lots of lightboxing.

{F1917111}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D16844