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
In my experience, It all depends on each theme.
Go to: Settings>Display and choose dark mode or light mode... depending on the theme, it should work on one of those settings If not, maybe try clearinf cache and data from whichever app you're using to download themes.