I can't change my background & form design

I can't change my background & form design

I cloned your form on my end and I am unable to replicate the issue. Would you mind telling us details of your issue? To customize your form design, please click on the "Form Designer" button in the right corner. 

You are able to find further options in the three tabs which are named as "Palettes", "Themes" and "Custom".

You can find available color options under the palettes tab.

You can find available color options under the themes tab.

You can customize your form under the custom tab. For example, you are able to add video background to your form.