Chameleon is just a boot loader, it does not have graphics drivers. It uses only basic functions exposed by your video card.
If the theme doesn't fill your screen:
Set the resolution of the theme itself in theme.plist so that it matches the native resolution of your display.
Set the Graphics Mode flag in /Extra/org.chameleon.Boot.plist so that it matches the native resolution of your display.
Try another video output, use an adapter if you have to.
Try another monitor (!)
If you have Intel onboard graphics use /Extra/Modules/Resolution.dylib.
If the theme still doesn't fill the screen, then either there is a bug in Chameleon, or the native resolution of your display is not available among your video card's VESA modes.
As a last resort, it may be possible to add the missing VESA mode to your video card's BIOS, some people have managed to do this with great success. There is a topic about this on InsanelyMac, we also have one somewhere around here I believe.