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