To choose a color click the background color tool and choose a type of color. Then, choose a color from the color palette.
You can choose colors from widget mode and design mode. In both places, you click on the background color tool.
Note: you can also choose background color of a menu. To see how to choose the background color click here.
You can choose color from the color palette either from the basic colors provided in the palette or you can also add the custom colors by simply providing the RGB or HEX value of the color in the input box.