Center Color - The center color represents the round splash that radiates from the center of the site to the edges. This splash is a gradient between the Center Color and the Background color.
Center Opacity - This slider allows the opacity of the Center Color to allow the Background color to show through and make the Center Color more subdued. Experiment with this setting and view the results on several non-calibrated monitors to be sure you have the setting you want. 65% works well as a starting point.