How to choose a unique title background image for each page ?

You can add a 'upload' meta option in 'include/page-option.php' . Add the following code at line #1110

"Page background" => array(
  'title'=> __('Page Background', 'gdl_back_office'),
  'name'=>'post-option-package-background',
  'type'=>'upload'
),    

and then you can use the wp_head hook to output the style in the header. Add this code in functions.php at the end before the ?> .

// Add items to the header
function add_custom_style() {
$bg_meta = get_post_meta(get_the_ID(),'post-option-package-background', true);
    if($bg_meta ){
        $thumbnail = wp_get_attachment_image_src( $bg_meta , $item_size );
        echo '';
    }
}
add_filter('wp_head', 'add_custom_style');

You should now see an option for the background-image in the 'Page Options'