First chroot, check ArchWiki for chrooting in btrfs.
Then run a system update by running garuda-update in the terminal .
Run the given commands in the following order :
1 sudo pacman -S linux-zen
2 grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=GRUB
3 sudo grub-mkconfig -o /boot/grub/grub.cfg
Then reboot .
The new grub menu will have the default black white theme.
How to theme it like Garuda, idk.