The easiest way the set a skin to RadSpell is by using the Skin="SkinName" property in the server tag of the control. As you can see on that page - http://localhost:5697/Live%20Demos/Spell/Examples/whatsnew/defaultcs.aspx
you may choose among a set of 11 predefined skins shipped with RadControls for ASP.NET AJAX.
If you plan creating a custom skin for RadSpell, please, have in mind that the control does not have it's own skinning, but relies on the skins of RadWindow (that makes-up the dialog of the spellchecker) and RadFormDecorator (that skins the controls inside the dialog).
More infotmation about how to control the visual appearance of RadControls is available on the following link: http://www.telerik.com/help/aspnet-ajax/controlling-appearance-overview.html
the Telerik team