Icon focus styles are cropped in Firefox on OS X

[sbates-idrc]

At 100% zoom, the keyboard and theme selector focus styles are cropped on the right and bottom. Daniel discovered that we can avoid this by using px for the focus style instead of rem units, but we need to test extensively to ensure this doesn't create side effects.

Original Jira issue: https://fluidproject.atlassian.net/browse/C2LC-441