Is it possible to custom css an individual page made with site builder?
Design and Skin Customization
Yes - each Site Builder page has a unique class added to the page container - i.e.
<div class="container sb-page-id-79">
Of course "79" is the ID and will be different for your page - just choose VIEW SOURCE and look for it.
So using CSS selectors - i.e.
.sb-page-id-79 .row {
background-color: #333333;
}
You can target any specific area of the page. I'd recommend using your developer tools and you will see all the different classes and ids available on the page.