To put it simply, my static site (made using HTML, CSS and JS) looks completely different when viewed from my custom domain. The text sizes become small and the background becomes all misaligned.
My vercel domain however shows the site correctly.
The domain troubleshooting guide can help with most custom domain configuration issues. You might be able to use that guide to solve it before a human is available to help you. Then you can come back here and share the answer for bonus points.
You can also use v0 to narrow down the possibilities.