Add dark theme option for makeovers

[fossfreedom]

Suggested enhancement.

Where a theme has a dark variant then we should have a checkbox next to the makeover button to allow the user to apply the dark-theme version of the makeover. If ticked, then the picture should update to the dark theme variant. Likewise, unticked then should show the light theme variant of the picture.

Obviously the tickbox should be pre-ticked if we default to the dark theme at the moment.