This help article will demonstrate a step by step tutorial how to customize the ControlDefault theme for RadDomainUpDown.
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 UpButton in Controls Structure on the left side. Then, select RadArrowButtonImage in the Elements section.
Modify the applied image repository item and select the desired image for the up button.
You can follow a similar approach for the DownButton.
Save the theme by selecting File >> Save As.
Now, you can apply your custom theme to RadDomainUpDown by using the demonstrated approach in the following link: Using custom themes