I have an s20 ultra and it's a new problem. Even when I do all that it'll go back to blank out of the blue. If I reboot it'll work for a little while than POOF, it's not working. I've had galaxy's for years now and NEVER had this problem. Text themes are important to me as I text so often and much of the appeal of the galaxy's
I could only get the icons & wallpaper from my theme...Go to settings, display, & make sure "Dark Mode" is off! After I turned it off the rest of my theme started working.