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, scroll down to the Styles section.
Here you can disable all styles (CSS files) included with WP Simple Pay Pro.
You can also alter the Payment Button, Checkout Button, and Coupon Apply Button styles
Example embedded form with Default styling selected for Coupon Code and Checkout Button
Example embedded form with Stripe Blue styling selected for Coupon Code and Checkout Button
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.