I would probably open a new thread to walk you through. Most changes are one liner commands. Grub customizer, well I have seen people posting here sporadically with unbootable system because they didn’t know what customizer did.
From what I can see in an older thread the grub menu is themed already by eos. Don’t know if things have changed since then or not but here is a post about it. Note normally you would just have to change that specific setting in grub config to my understanding and simply edit a text file and run mkconfig to regenerate/update grub configs.