There are couple of ways you can change the background color of the jmol canvas.
1. Right-click on the canvas to open context menu, in Color option, scroll down and select Background from the list, select the preferred color from the drop-down options.
2. Using Console window. Open the console window from the file menu, at the prompt type as command to change the background color. You type the name of the color in the command line or use RGB color code in square brackets as shown below (as an example to change background to red).