Let's say we have a a custom control named MyTextBox which derives from TextBox control and we need a skin for our MyTextBox control , we can do this by registering our custom control in the skin file
Ex : <%@ Register Assembly = "WebControls.MyControls" Namespace = "WebControls.MyControls" tagprefix = "cc1" %>
< cc1:MyTextBox runat="server" color = "black" backcolor="grey" />
Note: If your custom controls are in the App_Code folder then we can omit Assembly attribute while registering.
That's it ,