As documented via bug reports and online support requests, many users have difficulty learning that window decorations are customizable, because they don't notice or understand the little icon-only button in the bottom-left corner of each theme preview.
This patch centers the buttons and adds text including the theme name , making it obvious what the buttons are for.
Also, clicking on one of the buttons now automatically selects its corresponding theme, because configuring an un-selected theme doesn't make a lot of sense and could lead to user confusion once this UI is more obvious and widely-used.
BUG: 390245