I used in the past “Grub Customizer” but for some reason it doesn’t detect the Fedora partition, the reason could be that it’s an UEFI system though I’m not an expert in the subject:
Any ideas?
Generally, on Fedora Linux, grubby is the tool of choice for creating/modifying Grub menu entries. Followed with a grub2-mkconfig command to write the changes to the necessary locations. There is a package to re-install if you bork your efi setup, grub2-efi.
Now if your talking of themes, you’re going to have to go to the grub2 site for that since Fedora Linux does not have themes in it’s repo’s.
Can you tell me what could be the reason why the fedora partition doesn’t appear in that app?
Likely due to the fact there is only a shim used there to point to the actual boot location.
You should be very carefully when using grub-customizer, it can destroy a lot of your system and at the bad end your system could become out of order. Theming of grub-menu can be a lot of work (creating graphics, the right resolution of the menu etc.). I suggest let it be and live with an unthemed grub menu. That’s also Fedora.