@betro-hakala I don't even know what you want to be transparent. Also, if you are using Vanilla theme (not Persona) then the theme's options won't be in NodeBB Category options, those are completely different.
Could you send a screenshot of what you want transparent? That would help us help you.
Would you suggest what kind of background would be best for my oxide theme? https://szkolnecentrum.pl/topic/1864/money-makes-the-world-go-round-do-you-agree