Click on the color box underneath the preview of your current theme environment. [ Example ]
A Color Palette will appear on your screen. [ Example ]
Hover your mouse over the swatches to identify the color of your choice. With the left mouse button click that color to select it. (For advanced users: If you know the hex value of the color you want to apply to the background you may enter it into the hex value field and hit enter on your keyboard to apply it) [ Example ]
Click on Save to confirm your changes. [ Example ]