Improve visibility of the keyboard shortcut focus reticle
Summary:
This ended up being pretty hard to see, make it a bit easier.
Test Plan:
Focused things using the keyboard reticle.
Reviewed By: tomo
Reviewers: tomo, moskov, jungejason, tuomaspelkonen, aran
CC: aran, epriestley, tomo
Differential Revision: 483