Well, in this case, there’s not much we can offer you.
Prompting the users to upload different images for dark / light themes is not going to be implemented because most users won’t even bother as I described above.
Adding custom CSS won’t work in your case since you’re not an admin on the site.
If this is bothering you a lot, then please feel free to point the admin of the site you’re on to this discussion and we can advise them on what custom CSS needs to be added to fix your issue.