@Ronald-Fischer
I think the “differences” you are seeing are when the two backgrounds are in effect together–could this be right? For example if you double-click a word (or click+drag select an entire word), you get the “selection” background color as well as the Smart Highlighing color…at the same time. This looks like a third color, but it really isn’t. Of course, maybe I’m misunderstanding…
But the general answer…to this question… :
Can I somewhere change the colours of these backgrounds?
…is:
See Settings (menu) -> Style Configurator… -> Global Styles and then:
Selected text colour
and
Smart Highlighting
…adjust each one’s Background colour to your liking…