By default the Divi Theme displays a dividing line between the main content and the sidebar. It's possible to remove this dividing line using CSS, like so:

.container:before { background-color:transparent !important; }
.et_pb_widget_area_right { border-left:0 !important; }
.et_pb_widget_area_left { border-right:0 !important; }

Note that there are two cases in which a dividing line may occur (either on a standard sidebar, or on a sidebar module). This code the line in either case.