I have no access to a o365 environment at the moment but in the onPrem version if I open up the form settings there is a "Background color" property I can use.
I'd think this option is available for the o365 version as well isn't it?
Yeah you want Layout Settings as stated above - you'll have fun with O365 forms, I think different dev teams worked on separate parts of it, as in this instance you can set Hex value or form background but you can't for a label (you have to use CSS), and there are about 3 different colour pickers used each with different functionality.