Not sure what you mean by it’s very limited here - tbh, I know what you mean but I just disagree ;). You’ve got the entire material editor at your disposal. The way Slate renders things is simple, yes, so you will need an actual artist to make things look right.
I’m not an artist and materials are not my domain - that’s why the following button looks ever so slightly underwhelming:
This is 1 min of work, though:
The widget is an Overlay panel + border + button with a round green image.