Use relative path for the background picture in the grub theme

The absolute path was breaking grub on systems with btrfs subvolumes.

debian/kali-themes.postinst

@@ -51,8 +51,6 @@ if [ "$1" = "configure" ]; then
     # Copy grub theme to /boot
     mkdir -p /boot/grub/themes/kali
     cp -r /usr/share/grub/themes/kali/* /boot/grub/themes/kali/
-    sed -i -e 's|/usr/share/grub/themes/|/boot/grub/themes/|g' \
-	/boot/grub/themes/kali/theme.txt
     # Rebuild the grub configuration with our config changes
     if which update-grub >/dev/null; then
 	update-grub || true

share/grub/themes/kali/theme.txt

@@ -1,7 +1,7 @@
 
 # Global Property
 title-text: ""
-desktop-image: "/usr/share/grub/themes/kali/grub-16x9.png"
+desktop-image: "grub-16x9.png"
 desktop-color: "#2f5595"
 terminal-font: "Unifont Regular 16"
 terminal-left: "0"