style: Clean up lockscreen styles to improve HC support
Update to use button drawing where possible so login items inherit nice high contrast. Also cleans out some seemingly redundant css.
Notable changes:
- selected item uses :focus styles instead of filled blue--solves an issue with the "logged-in" indicator being invisible
- "Not listed?" button uses flat button style instead of non-standard label styles
- padding was adjusted to address the scrollbar overlapping user list times