Use ems instead of pixels for checkbox

Mixing pixels and ems results in a negative height, which is invalid.
Use ems instead of pixels for @focusBottomBorderSize.

Follow up: Ia6adc4bda000508f14acf4214e778118efa35be0

Bug: 71506
Change-Id: Ia619916a172a501365e0104e9927c7fae9de8a3f
Merge "Use ems instead of pixels for checkbox"