Hi Giovannino,
To customize the central part you can use CSS to hide the form white background and borders so the background image can fit with your custom design.
Custom CSS (replace .element with your real selector):
.element { background:none !important; border:none !important;}
Remember to use !important to overwrite all CSS rules of SC.
I recommend you to use chrome or firefox delevoper tools, for example in chrome you only need to right clic on a element of the page and select on the menu “INSPECT ELEMENT” to know the CSS class or CSS ID.