some themes make some highlighted or selected text very hard to see, and the previews don’t show that in advance: previews of themes probably should (if possible) include an explicit example of all forms of text highlighting, including headings, etc.
Have you tried opening an issue in the repositories of the themes in question? As a suggestion, you might want to tests themes in the help vault with the “Format your notes” note.
I’ve also edited the title to reflect the request.
Yes, I have, thanks.