I will add my answer since I don‘t use any of the other answers provided.
With the Support Library v7, all the styles are actually already defined and ready to use, for the standard buttons, all of these styles are available:
style="@style/Widget.AppCompat.Button"
style="@style/Widget.AppCompat.Button.Colored"
style="@style/Widget.AppCompat.Button.Borderless"
style="@style/Widget.AppCompat.Button.Borderless.Colored"
Widget.AppCompat.Button.Colored
: