There are a few options that allow you to change the styling your site visitors see when they’re viewing your payment forms.
In your WordPress admin under Simple Pay Pro → Settings → General (tab), scroll down to the Styles section.
Here you can set the Payment Button and Coupon Apply Button styles, as well as disable all styles (CSS files) included with WP Simple Pay Pro.
Note that there is no way to change the styles in the Stripe Checkout overlay at this time.
If you need to change styles even further, see Adding Custom CSS to WP Simple Pay.