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How to apply custom style in the WinForms SplitButton?
Customization of SplitButton
The appearance of the SplitButton control can be customized by using the Renderer property. The following code example demonstrates the same.
C#
//Initializes the CustomRenderer class. CustomRenderer renderer = new CustomRenderer(); //Sets the SplitButton custom style. this.splitButton1.Renderer = renderer;
VB
'Initializes the CustomRenderer class. Private renderer As New CustomRenderer() ‘Sets the SplitButton custom style. Me.splitButton1.Renderer = renderer
The following screenshot displays the customized SplitButton:

Reference link: https://help.syncfusion.com/windowsforms/split-button/appearance-customization#customization-of-splitbutton