Borders and margins option can be found by going to Settings > PDF Themes > click on a theme > Borders & Margins and it is where you can decide what parts of your documents to surround with a border, and what the margins should be.
For borders, use the tick boxes to select what parts to surround with a border – this will place these details in a thin black box.
For margins, enter in the spacing that should be applied to the document layout. A lower number will display the elements closer to the page edge. You can also use negative figures here to get the elements even closer to the page edge.