Get the background here: Board backgrounds library - #77 by yebellz
Set the lines to (224, 187, 108) for RGB values, or the hex is #e0bb6c (note: the color input widget is browser dependent and some will accept color values in different ways).
Note: also need to turn off the coordinates to get things to align. If you want to use coordinates, you need to make a different version that has a properly sized margin to account for that and manually add the coordinates to your background image (since the ones added by the site will be painted on top in the same color as the board, and actually that might interfere with you adding coordinates back in manually).