When I change the backgroundpicture in a theme it moves now together with the form itself

In the original sunset-hair theme the background remain on the same place and you could nicely scroll with the form

Hello robertoblanco,

That is because you have set the background image of the form section instead of entire page. I woudl suggest you to please delete the custom css code you have injected in form to change the background image and inject the following custom css code:

.supernova {

    background-image: url(https://www.jotform.com/uploads/robertoblanco/form_files/fb.977.jpg);

}

.form-all {

    background-image: none !important;

}

The following guide should help you how to inject custom css code in form:  https://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes

Hope this helps.

Do get back to us if you have any questions.

Thank you!