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 → Advanced.
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 are limited ways to change styles on the Stripe Checkout payment pages, see Stripe Checkout Display Options.
If you need to change styles even further, see Adding Custom CSS to WP Simple Pay.