Change background color

Sure, each form has a div "wrapper" element containing it and you can easily apply a background color to that.

This would apply the background color to all of your forms.

[css]
body .gform_wrapper { background-color:yellow }

and this would apply only to form ID #1. Note the _1 in the ID used below - you would replace that with your actual form ID

[css]
body #gform_wrapper_1 { background-color:yellow }

You can more information on targeting form elements for styling manipulation in the documentation here.

http://www.gravityhelp.com/documentation/page/CSS_Targeting_Samples

http://www.gravityhelp.com/documentation/page/Where_Do_I_Put_This_Code%3F

There are no default section "containers" like fieldsets. The section breaks are structured as a node in the same unordered list as the rest of the fields. If you want to wrap fields in actual containing elements be it fieldsets or divs, you will have to make use of the inline HTML fields to add your own markup. It's not really as difficult as it sounds and I've detailed how to do it here.

http://www.gravityhelp.com/forums/topic/how-can-i-get-the-form-fieldsboxes-side-by-side#post-29423

After you add your new containers, then you would target them by their unique ID and apply backgrounds or whatever other style properties you would like.

@MarcelFenchel You wouldn't change ANY of the Gravity Forms files. You should never edit the Gravity Forms files. If you want to change any style elements you would do so by adding custom CSS to your themes stylesheet to target and style the form element you want to style.

Kevin provided a link in a post above to the help guide for targeting and styling form elements. You can find it here:

http://www.gravityhelp.com/documentation/page/CSS_Targeting_Samples

We do not provide support for customized versions of Gravity Forms where a user has edited the core plugin files. All customizations should be done either via CSS you add to your themes stylesheet, or via the use of hooks and filters when it involves customizing something using PHP.