Let's say you want to change the look of one sign-up form but just that one and not the ones on all the other pages. You can do that by adding the page tag to some custom css.
If you don't already know, you can add custom css in the advanced section. See Can I customize my css?
But modifying one form is slightly tricky. Let's say you want to modify the form on this page: http://yourdomain/pages/6/signup-to-volunteer
In the above url, your page id is 6
To customize the css for the form only on page 6, you'd add this to your custom css file:
#display_6 div.form, #display_6 div.form form, #display_6 div.form form fieldset {
background-color: #000;
}
The import part is the #display_.... portion. You can add any css controls you want.
Once you have saved your custom.css, return to your page and refresh. Depending on your browser, you may need to do a semi-hard refresh (shift+f5).