How to apply the same CSS class name, from different CSS resources, using C and GTK4

Question

Still can't understand how are working CSS classes in GTK(4).

I have 10 CSS files. All these files have the classes with the same names. I need load these 10 CSS files at the same time, and apply style name to some GTK WIDGETs into window.

    css = gtk_css_provider_new(); // Create gtk css provider
    
    box = gtk_box_new(GTK_ORIENTATION_VERTICAL, 0.5); // Create example GTK widget. 
    gtk_css_provider_load_from_path(css, "file1.css"); // Load CSS from file1.css
    gtk_widget_add_css_class(box, "main_color"); // Adding style named main_color, to box
    
    box = gtk_box_new(GTK_ORIENTATION_VERTICAL, 0.5); // Create example GTK widget. 
    gtk_css_provider_load_from_path(css, "file2.css"); // Load CSS from file2.css
    gtk_widget_add_css_class(box, "main_color"); // Adding style named main_color, to box
    
    box = gtk_box_new(GTK_ORIENTATION_VERTICAL, 0.5); // Create example GTK widget.
    gtk_css_provider_load_from_path(css, "file3.css"); // Load CSS from file3.css
    gtk_widget_add_css_class(box, "main_color"); // Adding style named main_color, to box

The problem what I have - according to DOCS what I'm exploring, window might have only one CSS style - current. So after this block, all 10 widgets (boxes in my example) will have stylesheet from file10.css, as last file which was loaded.