Installation script doesn't seem to work for Debian 13

I tested it on Debian 12 and it works just fine but on Debian 13 theme didn't change despite `GRUB_THEME` has been changed in `/etc/default/grub` and theme has been installed  to `/usr/share/grub/themes`. It could be also related to the fact I have `grub-btrfs` installed but it shouldn't - it's just one extra  menu in grub.