To change GUI colors using the Appearance Window, you need to click on the color button next to the color that you want to change. To change the background color (the color of the GUI's windows), click on the GUI background color button and select a new color from the Popup color menu. To change the foreground color (the color used to draw text), click the GUI foreground color button and select a new color from the Popup color menu.
VisIt will issue an error message if the colors that you choose for both the background and foreground colors are close enough that they cannot be distinguished so you do not accidentally get into a situation where the controls in VisIt's GUI become too difficult to read. Some application styles, such as Aqua, do not use the background color so setting the background has no effect unless you change to an application style like Motif, which does use the background color.