Hmm, I am not sure what you mean. In my online chess games the board is a wood/brown color. If you are talking about chess boards in the forums, articles, and blogs then the user who actually creates the board can choose the square color and size through the game editor, but unfortunately the observer of the board cannot change the color or size.
This brings up an interesting debate though, which color scheme and piece set is the best for Chess.com boards. Maybe this can be a future survey? 
Can the default chessboard be changed, from the dark blue squares to perhaps to wood or perhaps gray? Because the default dark blue sometimes makes the black pieces/pawns invisible. The wood/brown color is great. If it is also possible, use the book pieces/pawns as default. And how about making the small chess board the default (the medium is still big)...