This help article will demonstrate a step by step tutorial how to customize the ControlDefault theme for RadDesktopAlert:
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 AlertWindowTextAndSysemButtonsElement in Controls Structure on the left side. Then, select AlertWindowTextAndSystemButtonsElement in the Elements section.
Modify the applied text repository item.
You can see the result directly in the Visual Style Builder.
Save the theme by selecting File >> Save As.
Now, you can apply your custom theme to RadDesktopAlert by using the demonstrated approach in the following link: Using custom themes