Below are some basic style changes you could use to update the look of the trivia app. With CSS, the possibilities are nearly endless. Just look at this CSS reference at w3schools. It is easy to just try things out by trial and error. Most styles have several options to consider. So, as needed, do a Google search for "CSS" followed by a simple description of a style you are interested in, and you will likely find some answers.
In your style.css file try changing or adding the following background properties to different elements in your trivia game,--like the body, .screen, or button CSS selectors to see the result.