How can I change background colour

What background color do you want to change? The form itself or the inputs? If you can post a link to your form, that might help too.

this will apply to the form wrapper

body .gform_wrapper {background:#f00}

or this to just the actual form element inside the wrapper

body .gform_wrapper form {background:#f00}

If you only want to change one form, you would use the form ID - replace the XX with your actual form ID.

body #gform_wrapper_XX {background:#f00}

I can't tell you for sure. It all depends on your theme and how it's built. Usually, it's style.css in your theme folder. Sometimes it's called layout.css or main.css. Themes like Thesis and Headway have a place in the theme admin to put custom css rules and some theme developers like Woo Themes have a custom.css file where you're supposed to put custom rules.

If you're not really sure, I'd suggest contacting your theme developer and asking.