Colour Changes

I was on this one site and when i clicked on a forum and opened it it would change the colour scheme and pictures on that page and all threads and forums inside that one.

How do i do this?

Are you talking about multi-themes?
https://help.forumotion.com/updates-and-new-features-f1/users-tuesday-tip-add-multi-themes-to-your-forum-by-darren-t71484.htm

Thank you. I hope this is what im looking for and when I get my laptop or a computer I shall try this out.