Both TelerikMetro and TelerikMetroBlue themes use the BackgroundShape property of the ActionButton's FillPrimitive to style the application button. In this sense, the green rectangle and the Telerik logo are inseparable through code. I have logged this issue to our Public Issue Tracker under ID 12586. To subscribe for its status update please follow this link: http://www.telerik.com/support/pits.aspx#/public/winforms/12586. We will resolve this issue in a future release. Your Telerik points have been updated.
Until the issue is resolved please use one of the following:
1. The attached sample project contains a version of the TelerikMetroBlue theme in which the ApplicationButton is only a green rectangle without the Telerik logo.
2. Use the following code to remove the BackgroundShape and set appropriate border and fill colors: