If you enable “Use Background Color” and “Ligh theme” then only the “Use Background Color” will works.
If you want only use a light or dark theme then enable ONLY the “Light theme”.
If you want use other colors as light or dark, then choose a background color and enable “Use Background Color”. Don’t enable now “Light theme”.
Btw:
Hover with your computer mouse on a block to see if the wanted dialog supports the new options or not.
The default “App Inventor” blocks dont’ support the new options.