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.
The Use light background setting is now strictly about the
background colour and won't affect the selection handles anymore.
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.