How do I change the background colour of OP posts and my posts?

[ktsa4]

I've been in the settings and can't find it anywhere, can anyone point me in the right direction?

Still can't find it, anyone around? I feel so daft!

I didn't know you could do this but I'd like to know!

I used to have an account years ago and back then you could, but clearly a lot has changed since!