Style The Application Button
The default look of the application button is an empty rectangular button similar to the one on the next snapshot:
When you want to set a picture in this button you can use the ApplicationButtonImageSource property. The result may look like this
<telerik:RadRibbonView x:Name="xRibbonView" ApplicationButtonImageSource="Images/3dRedButton.png">
...
</telerik:RadRibbonView>

Also for further customization you can set the following properties:
- ApplicationButtonContent
- ApplicationButtonStyle
- ApplicationButtonVisibility
- IsApplicationButtonEnabled