Hi!

I come back here to answer my own question.

( thanks to Codigo for helping me on the forum)

To add a background image, instead of the beige-white background.

you must replace the contents of the named section
/* --- Content Background --- */

by :

I’m writing it here for you if you want to copy and paste:

.row-offcanvas {


background-image: url('url here');

background-attachment: fixed; /* --- I use a gradient background, it allows to display it without the patterns being huge (I use images of 4000px) --- */
background-size: cover; 
background-position: center;
}

Have a good day !