Themes
This help article will demonstrate a step by step tutorial how to customize the ControlDefault theme for RadCheckBox and change the back color of the checkmark:
- 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 
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Select RadCheckBoxElement in Controls Structure on the left side. Then, select CheckMarkBackGround in the Elements section.  
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Modify the applied fill repository item.  
- Save the theme by selecting File >> Save As. 
- Now, you can apply your custom theme to RadCheckBox by using the demonstrated approach in the following link: Using custom themes