To choose a new color for the board's dark squares, choose Settings > Board > Dark squares..., choose a color, and click OK.
This option colors all of the board's dark squares in the shade of your choosing.
If you're setting this option from the command line, you will need to pick a valid color name. The program recognizes many common color names, such as red, light blue, ivory, and more. See a full list at the TCL web page. See all the colors represented by these color names at the TCL Wiki. You can also specify a hex code such as #0f3296 (a darkish blue) or #00FF00 (bright green). Remember to include the hash or pound symbol, "#", at the beginning of the hex code. If you specify an invalid name, Chess will use the default dark square color, "gray35".