How to apply CSS on a form page?

I successfully created a contact page. The only problem i have is, that the input fields use the whole screen width. I'd like them to be in a centered box in the middle of the page. How can i do that?
I already tried to add a <div> to the corresponding form.html.twig but that doesn't work. The twig seems to be only applied to the title of the page. Suggestions?

I'm using this to set a maximum width of 24rem and center all forms on my site:

---css
form {
max-width: 24rem;
margin: auto;
}

Thanks a lot! :D