hello , how to change the background from white form ?

Sorry i dont know!
This form is based on another extension. You have to analyze the form with Firebug and then add the found css code into the file:
css / custom.css

Video | How to customize a theme with Firebug
www.joomlaplates.com/template-customizing

I received this response from the support JomSocial:

Hi, you can put this CSS at your template custom CSS

.joms-popup__content {
background: #000;
}
please asking your template provider about how to put custom CSS.

You can add your custom code to the file: css / custom.css
If the file is not existing, create the custom.css