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.
I was having the same issues and took my phone off of dark mode, suggested earlier in this thread, and it solved my issues completely! I have a Samsung Galaxy A11 for clarification. Everyone should try this first before attempting more complicated solutions. Thanks whom ever suggested the dark mode off idea it was the solution for me!
Guys, try leaving Dark Mode. Earlier this year I noticed that switching on dark mode cancels the themes' effect on the messaging app (the stock one, obviously) and the call/contacts screen.
It just fixed the problem for me after I went as far as performing a factory reset today. Then I remembered what happened earlier this year and simply went back to light mode and voila!! Problem fixed!! I must've switched dark mode on, forgetting it cancels the themes' effect.
Hope this helps.