Detect dark theme by '-dark'
The default supported themes to my knowledge are Adwaita and Adwaita-dark.
As far as I can see, we don't have any way to really know whether a
theme is dark or not. I would like to suggest to check for -dark
instead of dark
. Assuming that it's more unlikely to not by accident
draw something dark that might be a regular theme name, like noahsdark.