need help Customise CSS with pro version

Hi,

I’m using Divi template with the WP Simple Pay pro version. But i just don’t get it how you customise the button.
Is there a way to overwrite the css of the button on a page scale ?
i allready had a look here https://wpsimplepay.com/docs/misc/calling-stripe-checkout-externally/ but it just don’t get it
Any simple css code i can push on top with some !important ?
Thanks a lot for your help.