To do this navigate to:

The category of the Item you are looking to edit for example a Text Category.
  1. Click the Editing Pencil on the Text Category
  2. Click the HTML button
  3. Find where it says 
  4. table border="0"
  5. Change this to table border="insert number you wish to use to make the border width" for example table border="1" or table border="5" (depending on how thick you would like the border to display)
  6. Click Save