garuda-boot-options is a decent app that allows manipulating various grub settings graphically
However it does not list any new themes installed from AUR or pacman .
Since they dont paste the theme in /boot/grub/themes ,
It would be more convenient if the app would list every installed theme from /boot/grub/themes and /usr/share/grub/themes/ in a dropdown list
/usr/share/grub/themes is the loaction where all manually installed themes are present
You can navigate to any folder you like. If you wish to continue storing your special grub themes in /usr/share/grub/themes, just navigate to that folder.