Try tabbing through the widgets demo.
Taken from merged-in duplicate T162891 as of v0.20.x:
In the MediaWiki theme, it's very obvious: there's a thick blue border when the widget is focused:
But in the Apex theme, used by MediaWiki's Monobook skin, it's so subtle as to be almost unnoticeable unless two identical buttons are right next to each other:
The difference is apparently only #eee to #aaa on a 1-pixel-thick border that's adjacent to other shades of grey.