In order to achieve the desired appearance of RadDesktopAlert you need to modify its default Template. So, basically you need to create a custom Style for RadDesktopAlert based on its default one. After that you need to place the desired Button inside its ControlTemplate with key x:Key="RadDesktopAlertTemplate".
We have created a sample project that demonstrates that. Please, check it and let us know if it worked for you - notice that Implicit Styles
We have also noticed that you have asked the exact same question on several other forum threads. We have provided the answer to them as well in order to be available for the other members of the community. However, please lets continue our discussion in the current forum thread, so that we could follow the communication more easily.
Hope this helps.
Telerik by Progress
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