Background color is not wide enough

I have tried changing width to 1500 px but it didn't change.

You can add this custom CSS code if you want the background image in the whole page.

.jotform-form {

background-image: url(https://cdn.jotfor.ms/images/styles/style2_bg.jpg) !important;

}

.form-all {

background: none !important;

}

Here is a guide on how to Inject Custom CSS to your form:

https://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes

Please inject the following code as shown in this guide: https://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes 

.supernova,.form-all {

    background-image: url(http://www.jotform.com/images/styles/style2_bg.jpg);

    background-repeat: no-repeat;

    background-attachment: scroll;

    background-position: center top;

    background-repeat: repeat;

}

Here is the result in my clone version of your form: https://form.jotform.com/73476721126962