To add a dividing line between the individual widgets in the Divi Theme's sidebar, we can add the following CSS:

#sidebar .et_pb_widget { 
    border-bottom: 1px solid #ddd; 
    padding-bottom: 20px; 
    margin-bottom: 20px; 
}
#sidebar .et_pb_widget:last-of-type { 
   border-bottom:0; 
}

The first part adds a bottom border to each widget (the same color as the vertical sidebar divider), and positions it mid-way between successive widgets. The second part removes the bottom border on the final widget.