If you want to add a line to one of your registration pages, navigate to that page in the 'Page Editor'.
In the menu bar, select the location where you want to have the line based on the item. (In the example, I want to place the line in the low of the Personal Data Delegate title.)
In the editor, click on the line symbol to insert a line and click 'Save'.
Now your page contains a line.
To add a colour to a line, go to this page.