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This is not possible with the current implemnetation of RadMessageBox. The buttons are not part of the control template. Instead they are created in code so that we provide the option to add custom number of buttons depending on the buttons content that you provide. In order to alter the look of the buttons, you can add a Style for the Button type to the resources of the page/application. However this approach would lead to having the same style for all buttons and you won't be able to specify which one to be used by the each of the buttons.
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