This help article will demonstrate a step by step tutorial how to customize the ControlDefault theme for RadTrackBar.
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 TrackBarThumbElement in Controls Structure on the left side. Then, the TrackBarThumbElement in the Elements section will be automatically selected.
Modify the applied TrackBarThumbFill repository item.
Save the theme by selecting File >> Save As.
Now, you can apply your custom theme to RadTrackBar by using the demonstrated approach in the following link: Using custom themes