black background while viewing flv files

Maybe someone can help me out with this.
I'd like to create a black background while viewing downloaded YouTube .flv files with the Applian FLV player. Because the image of a .flv file is rather small a black background to fill up the rest of the screen is more beautiful to watch. Windows XP, f.i., creates a black background if you watch your photographs in a diashow.

Thanks for any help.
Antonius, Eindhoven.