To style the buttons, the app allows you to add your theme's CSS classes to each button to perfectly match your theme. To do this, you should find the button on the storefront that you want to copy, then right click on it and go to "inspect element". There, find the btn element then look for the text following the attribute 'class='. Then simply add that class or classes to one of the buttons in the 'Add CSS classes to popup buttons' section. Then the button in the popup will look like the button you copied the class from.
This video shows you how to find the CSS classes for your store's buttons.