It is because it was chess.com many years ago. They updated to the transparent ones now. As well as arcade, instead of lighting, you get like a colored square
For chessbase and lichess you can change the color by holding ctrl, shift, alt, or some combination of 2 of the keys.
I haven't tested this with chess.com though.
Chess.com can
It is because it was chess.com many years ago. They updated to the transparent ones now. As well as arcade, instead of lighting, you get like a colored square