This help article will demonstrate a step by step tutorial how to customize the ControlDefault theme for RadVirtualKeyboard
Open VisualStyleBuilder: Start menu (Start >> Programs >> Telerik >> UI for WinForms [version] >> Tools).
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 ToggleKey in Controls Structure on the left side. The ToggleKey in the Elements section is also selected.
Expand the ToggleKey Element and modify the applied BackColor property and set the GradientStyle to Solid for the ToggleState=On Element State.
Save the theme by selecting File >> Save As.
Now, you can apply your custom theme to RadVirtualKeyboard by using the demonstrated approach in the following link: Using custom themes