You know there's that one property you don't understand
Seriously, there probably is a property that you don't get, even if you understand them all, always nice for a recap!
Colour Theory - do you know it?
As it turns out, doesn't really matter if you do or not. It's the modification of those colours within an interface you should be focusing on.
How do you even begin?
Well, standardisation. That's the long an short of it. Pick spacing, colours, a type scale, and then stick to it. Work exceptions in as and when. Don't kid yourself - there are always exceptions.