Improve .expandable_target focus styles

Sorry for the extra commits. Needed to get synced up with the upstream or something.
That's the horizontal padding you're citing. I tried the standard 22px vertical padding, but it looked really weird because the vertical padding on `.expandable_header` is only 10px. I just lazily changed it to 15px because that looked decent in the inspector. I kinda feel like either going to 16px (to match the horizontal padding) or 10px (to match the `.expandable_header` vertical padding) would work. Do you have a preference between those two?