I still prefer Syslinux but it's still not officially supporting UEFI yet :(

The only time I would want Grub is for that reason primarily. But it makes sense to be very familiar with Grub so you can not be screwed up by systems you encounter which may have been set up, "grub-reinstalled", or configured by those who are not as familiar as they should have been.

Once you do have a regular ordinary multiboot system or SSD that includes one or more Linux installs, with Grub working well, then adding the themes like this is only the icing on the cake.

Does LILO work on EFI systems or is GRUB preferred in that case? I’ve not built a Slackware box in a long time… maybe this evening I’ll see about putting a VM together

Slack ships elilo package for EFI systems. AFAIK config (elilo.conf) should be pretty much the same as standard lilo.conf

I can’t think of a single reason to entertain Slackware or lilo. I have extensive experience with both.