@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.
Is it possible to add a chat room?
Is it possible to add a chat room? Everyone can speak there publicly like shoutbox https://community.nodebb.org/topic/503/nodebb-plugin-shoutbox-shoutbox-plugin
There is a global chat plugin: https://github.com/NodeBB/nodebb-plugin-global-chat