You cannot directly control the MessageDialog's appearance. The colors will come from the background color set in your app's manifest. From
Guidelines and checklist for message dialogs
Message dialog backgrounds are always white. The primary colors of the app that owns the dialog are used for controls.
If you want more control over the dialog then you will need to use a custom implementation rather than the system provided dialog. MessageDialogs should be used sparingly in any case as they are generally not the most user-friendly way to provide information.
See
Guidelines and checklist for message dialogs and
Adding message dialogs for examples of when to use MessageDialogs and when to use other methods.
--Rob
