Add an option to use the system theme
It would be cool to have the option to use the system theme.
Instead of using a slider button to switch between light and dark theme, an
ExpandableListItem with the options Light, Dark and System could be used. (Already done here: https://gitlab.com/ubports/apps/clock-app/blob/master/app/alarm/AlarmSettingsPage.qml#L227 but unfortunately the clock doesn't know how to query available themes.)