allow different backgrounds in light and dark themes

Cosmic Epoch has light and dark themes. If you set the background to be a bright picture, and then switch to the dark theme, it clashes. I would like to be able to set separate backgrounds for light and dark themes.

One inspiration is Night Theme Switcher, which I used to switch backgrounds in GNOME on Pop OS: https://gitlab.com/rmnvgr/nightthemeswitcher-gnome-shell-extension