This article will show you how you can make a line in you banner by using a gradient asset.
1. Start by drag and drop any of the gradient assets to the canvas. You can make it fill the whole canvas by double clicking on the gradient.
2. Adjust the height of the gradient until you are happy with the thickness. By dragging it all the way down you will get a 4 px line.
3. Since you probably don't want your line to have a gradient, you can remove the gradient style from the styles panel and instead add a background color.
4. If you want the line to be thinner, just mark the line and click on Position & Size and then adjust the height.