I would also like to have an option for themes to leave my Windows background color alone -- even if the end-user has to modify the theme to accomplish that, e.g., by setting the background color for the theme to "null", "indie", "please just leave it alone", or whatever.
Alternatively, if it's easier coding-wise to stick with the current color-selection popup, please add a color-picker widget so that the end-user can select the background color they wish to use from the background color they have already chosen using the Windows "Personalize" popup.
Hoping the developers might consider one of these possibilities.
Meanwhile I suppose it's not a great hardship to keep changing the background color back to what I want using "Personalize" every time that needs to be done.
Thank you for your consideration.