This help article will demonstrate a step by step tutorial how to customize the ControlDefault theme for RadCommandBar
Export the built-in themes in a specific folder by selecting File >> Export Built-in Themes.
Load a desired theme from the just exported files by selecting File >> Open Package
Select CommandBarStripElement in Controls Structure on the left side.
Expand the CommandBarStripElement in the Elements section. Modify the applied back color and gradient style.
Save the theme by selecting File >> Save As.
Now, you can apply your custom theme to RadClock by using the demonstrated approach in the following link: Using custom themes