I had to figure it out for myself because people keep it a S E C R E T and it get s annoying so imma tell u

You use the paint bucket and paint it whatever your BACK round color is you move it to the side to but not completely and line it up with the top and bottom . Then you make the thickness of the line. Then use paint bucket to color in what color you want.then you make a new page on a page? Idk what they call it but make it the same color as the first color you chose for your base. there,now do wth you want with your line