As I said, I'm not displaying the titles. My concern is the background color of the tiles themselves, and I haven't found a way to change those.

But nevermind, I just found out that the search engine choice of this extension is weirdly limited, so I will drop it anyway.

Doesn't have any effect (after all, I'm not even displaying the titles).

Yes, I set it to a solid color, but it's the same color for the overall background, as well as the individual tiles.