LiveCD: set Baby Bee wallpaper centered on black background

400×400px PNG centered via feh --bg-center --image-bg '#000000'.
Fallback solid fill also changed to black.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/overlay/usr/local/bin/bee-openbox-session

@@ -9,9 +9,9 @@ xset s noblank
 
 # Set desktop background.
 if [ -f /usr/share/bee/wallpaper.png ]; then
-    feh --bg-fill /usr/share/bee/wallpaper.png
+    feh --bg-center --image-bg '#000000' /usr/share/bee/wallpaper.png
 else
-    xsetroot -solid '#f6c90e'
+    xsetroot -solid '#000000'
 fi
 
 tint2 &