"Welcome to GRUB!"

This isn't intended. If this only appeared after using grub customizer, then it may be a configuration issue (removed some "script code" stuff, for example [use the "show placeholders" toggle from the menu to find out]).

The revert button on the topright of the menu list configuration should fix this as well (reverts your list configuration).

This is what I see when I use "show placeholders". But my knowledge of grub or the customizer isn't that deep, so you'll have to help me here a little. Should I remove any of the placeholders BEFORE the list of OS?

https://i.imgur.com/APcW0VD.png

Looks good. The only thing you can try, is to restore the default configuration using the button on the top right. If is works, then we have to compare the resulting /boot/grub/grub.cfg with and without modification.

If you're talking about the "restore" button, it only brings back to the menu the options I removed but it doesn't change the presence of "Welcome to GRUB".

I just found out that this message is in GRUB's source code, so apparently it's intended to be there (not that anyone wants it, tough) but it still remains a mistery why it appears only after using GC to alter grub.cfg.

Ok, then I have no idea, what's the reason for that extra message to come. Generally, grub customizer isn't changing things without a reason. So if you're just opening grub customizer and are pressing the save butten, then the resulting config should remain the same as before.

No, I've added a background image and changed the colors, aside from removing the unnecessary options.