Ubuntu Software gets into a transparent background

Same issue here on 20.04. Fixed it! Go into settings -> appearance -> and select something to your preference in "Window colors"

Ubuntu Software is now a Snap. Snap programs do not support nonstandard themes (meaning the ones you'd have to select with the Tweaks program rather than regular settings; --there can be similar issues with Flatpak) except ones also installed via Snap. The new Ubuntu software also no longer supports Flatpak.

What I had to do is install the packages "gnome-software" and "gnome-software-plugin-flatpak", then just stop using "Ubuntu Software" and use "Gnome Software" instead.

I am having the same issue 20.04 when I change the theme to any theme downloaded from gnome-look.org. The themes I have tried are compatible with the Gnome version.