Graphical GRUB Menu

Try BURG. I'm sure this is exactly the ticket. It is a themeble GUI overlay for GRUB & Grub Customizer can still administrate.

Brand-new Universal loadeR from Grub;

burg is a brand-new boot loader based on GRUB. It uses a new object format which allows it to be built in a wider range of OS, including Linux/Windows/OSX/FreeBSD, etc. It also has a highly configurable menu system which works in both text and graphic mode. Additional features like stream support and multiple input/output device are also planned.

But, yes, as of comment, not actively maintained. Still I am not aware of an alternative, lets hope a developer picks up the slack...