Single post in A Somewhat Extensive Guide to BBCodes

Hello there! I hope this is the right place, but I'm wondering how you recolour things in posts? I'm wanting to make some sort of shiny Porygon theme for the shop I'm working on, although I can't seem to figure out how to change the colours of the tabs, displays, background, etc. Hope this doesn't sound dumb! I'm pretty new to coding on here.