I'm using one of html5up.net's templates and when I add a stripe 'Pay by card' button, it's overridden by the css of html5up (see both jsfiddles below). I tried going through the css file and see if there's anything I could change to affect the button's appearance to no avail (the file is absolutely massive and I could use some steering from more experienced heads).
Jsfiddle of how the button is supposed to look like (no css loaded)
<form action="/your-server-side-code" method="POST">
data-key="a public key"
<link rel="stylesheet" href="main.css" />
Line 1897 of your CSS is describing styles applied to the buttons.
If you remove line 1923:
It will look kind of ok (or just delete whole section from 1897 to 2050 and you will be fine).