this is how i edited my zone.xml file to have the white border around the play area of my map. create a new zone. make the new zone around the play area of your map, i.e. where you want the white border. save the map and exit. go to zone.xml and change
<borders tl="-10000 10000 5000" br="10000 -10000 5000"/>
to whatever coordinates that zone you just created has. go back to map editor and the white border will be where you wanted it.