I’m brand new to the Watch Face Studio and it seems to default to a black background (which just may be the background for the studio itself.)
I tried adding theme colours but it seems the only thing I can do is add an image of a white background. Which is fine but then the build fails as it says for Always On the OPR is too high?
I’m not sure if any of those docs answered your questions.
You can start with any color background or use an image. I like the image-cgradient_pale.png as it adds gradient to my watch face background.
Then with the background selected go to Style and add other colors for user customizations.
Black is the lack of color White is all color so if you have a white background in AOD Mode it will fail the On pixel ratio. either turn the opacity down so it is darker or swap image to something that is paler.
When you design the AoD be sure you are working in the AOD tab. There is a show/ hide for each layer you can just hide the Normal background in AoD.