The button RadSpell renders is a regular button or anchor (depending on the ButtonType property). Since it is not a Telerik control it cannot be skinned directly.
What I can suggest is going with one of these options:
- use a RadFormDecorator to style such elements on the page: http://demos.telerik.com/aspnet-ajax/formdecorator/examples/overview/defaultcs.aspx
- hide the built-in button, add a RadButton and in its OnClientClicked event
(note - AutoPostBack should be false) invoke the spellcheck through the client-side API of RadSpell: http://www.telerik.com/help/aspnet-ajax/spell-client-side-api.html
(see the startSpellCheck() method).
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