Prompt button classes

[boulepick]

Hi there i noticed that i cannot change the default colors for the prompt buttons. using the Bootsrap theme and it always default to btn-primary for the OK button and btn-secondary for the cancel button. We should be able to define the button classes so that i can make a button green with the btn-success class and red with the btn-danger classes for example or any other classes of choosing.

thanks for the great work