You can style a lot about the combobox by changing the generated widget it uses for each item in the list,
Next fill in the created function with whatever widget you’d like to generate, in this example I made a widget called PlayerEntry that’s just a textblock formatted the way I want, that has a spawn parameter for a players name that I pass in, on construct it sets that text on the textblock it owns.
The specific option to style the background is as far as I know unsupported in the slate control, so unless you modify the underlying slate control, or make your own combobox, there’s not currently a way to style it. My door is always open on pull requests 
The UMG Team is really tiny. Me and occasionally someone else, so not a ton of free time.
I tend to wait and see if someone else will answer the ‘how do i style’ posts because I figure someone else has figured it out. Also if it’s not built in, you can always mod the engine or clone the control and get generally whatever effect you need - so there’s always a workaround.