“Fixed(?)” (I don’t know if you can call this a proper fix):
I remembered that I also had Grub-Customizer installed at one point and I followed this post:
su
rm -rf /etc/default/grub
rm -rf /etc/grub.d
exit
sudo pacman -S grub
and then I added GRUB_THEME="/boot/grub/themes/EndeavourOS/theme.txt" to /etc/default/grub/