In the iframe that has the Give form, there’s this line, repeated twice:
<style id="alpha-critical-css">body{opacity: 0;}</style>
All you need to do is to remove it for it to work. Does that ring any bells? If it’s hard to figure out, you could try searching the codebase and see where alpha-critical-css is coming from.
By the way, I found this just by deleting sections of the CSS in the browser inspector until it was fixed.