After doing this exersize, I am able to set button backgrounds, but I am struggling to apply what I learned to do an actual styling in a real-life situation because the tutorial does not mention the current capabilities / restrictions of the styling.

For example, setting a Button background (as done in the tutorial for Composite) works well, but I cannot find any resource nor a hint in the css spy on how to set the style for a selected, hovered over, default button, or disabled button.