How to save a desktop theme with picture and selected background colour

I’m trying to save a theme for my desktop. I have a picture which doesn’t fill the screen, but I want the rest of the screen to be the same background color:

(I have a different background colour above to show what I’m trying to do).

I first set the image, center it, and then choose a background colour.

When I save the desktop theme, either locally or for sharing, it only includes the image, but not the background colour, so the rest of the background is white.

Am I missing a step to save both?