As a follow-up to 4e2f90a30b66, in addition to using the system's color
for the text labels, we now also support user-chosen colors for the
label's background and border. The highlight color for the selection
handles and the magnifier is improved, too.
Note: Due to the opacity, we have to use `systemPalette.light` instead
of `systemPalette.window`, otherwise for light schemes the contrast
would be too low.