Hi Lucie. Thanks for including that link.
The issue is that your theme has styling that is affecting form fields. Here’s a video demonstrating this: https://cl.ly/3h3p3Z1Q1u2D
You’ll need to determine how best to overcome this issue with your theme, but typically it’s solved by adding CSS rules with more specificity to make things look how you want them.
If you need some coding help, you may want to ask your developer or reference our list of known customizers.
I hope this helps point you in the right direction. Please let me know if there’s anything more I can help you with.