Lines can be added to your cover or spine design using the 'LINE' option shown on the menu at the bottom of the Cover Design Tool panel.
Drawing A Line
Click the 'LINE' icon and draw a line by clicking and dragging the mouse.
Select the line and change the line colour using the 'STROKE' option.
Change the line width using the 'STROKE WIDTH' option.
Drag the slider from 1 to 50, with 1 being the thinnest line and 50 being the thickest.
Duplicate a line using the copy and paste options shown in the menu at the top of the Cover Design Tool panel.
Select the line, then click 'COPY' and 'PASTE'.