Hi, yes, becuase of width of container.

But anytime you can add bg image using custom CSS, just use simple CSS.

For example set BG Image for that ID

#sp-main-body { }

and before you would ask me how? Read:

• https://www.w3schools.com/css/css_background_image.asp
• https://developer.mozilla.org/en-US/docs/Web/CSS/background-image

have a code to add? I'm not able with css...

Oh.... we cannot teach CSS here for obvious reasons :/

For example

#sp-main-body { 
  background-image: url("images/background.jpg");
    background-repeat: no-repeat;  
    background-position: top; }

rename file name used in my example to current one, images/Foto-sala-per-sezione-Teatro-1.jpg

https://www.w3schools.com/css/tryit.asp?filename=trycss_background-image_position

there are tons of guides in Italian language, you can still learn new things.

OK, I will check what you did wrong.

Now is working fine, you have bg image on subpages. I guess you updated CSS.

but please improve (increase) contrast between bg color and font color, now is TOO LOW.

Think about accessibility - also older people with weak eyes. It's hard to read for them.