This patch updates the "Theme" tab to react to an override theme being
set. The preview will reflect the override theme and the combo box will
show no selection.
Previously the "Theme" tab in DisplaySettings would only reflect the
current theme on startup and get out of sync when a theme would get
selected using the taskbar menu.
With this change you can now set the theme and background color at the
same time in the Display Settings. Before if both were changed
before hitting 'apply' the theme background color would overwrite
the custom background.
