Hi @monjoro, and welcome!
Hehe, while I’m not a designer, I know how you feel, always wanting to customize, to tweak and in general just…rice’ing it.
I have a grub theme myself, and I got the inspiration from this thread, so it’ll possibly be inspiration for you as well.
What’s more, I’m now strongly considering swapping my grub for rEFInd. But I’m still reading up on it, on the procedure of going about it and then I’ll make a final decision on it.
But I hope that helps at least a bit.