Wow. I got a 21 by just clicking the first 16 in a row and then doing a scattershot. I don't think I'll beat that, so I'll post that as my best score ever and walk away!
Made the change. (All I added was .grid-item.red to the CSS lines, which targets the incorrect answer that was clicked i.e. the one with the solid red background after the quiz.)
Me after taking the quiz: Never mind