To change border picture you have to go to:

Steam\steamapps\common\Brave Battle Saga - The Legend of The Magic Warrior\res

Here you have to find "back" file without file type, open it in any image editor (paint, e.t.c.).



Then change it to black without changing resolution (In "Paint" select all with ctrl+a and delete it, then fill with black, then save. And it's done.

With this method you can turn this:



Into this: