The background globe 
on the toolbar launches the Background Settings window.
You can choose between a Solid color or a Skybox.
To set a custom background color, select Solid color, click on the color field, and select a color.
To set a skybox, select Skybox and choose from the Day, Evening, and Underwater options.
Be mindful that inverted x-ray works best with light backgrounds and regular x-ray with dark backgrounds.