This help article will demonstrate a step by step tutorial how to customize the ControlDefault theme for RadRibbonForm.
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 RadRibbonBarCaption in Controls Structure on the left side under RadRibbonBar. Then, select RadRibbonBarCaption in the Elements section. Modify the applied BackColor property.
Select RadRibbonForm in Controls Structure on the left side.
Save the theme by selecting File >> Save As.
Now, you can apply your custom theme to RadRibbonForm by using the demonstrated approach in the following link: Using custom themes