Ah cool I see it works with themes too. Ah well. ;)

@hurricanefan

Trust me I tried all of that, wouldn't work at all. Worked fine for me before retropie was updated but not since. I've put them in the splashscreens folder in supplementary and works perfectly now

IIRC there is no root password.
Single user mode won't help as it requires root password too, and you need to be root to edit /boot/cmdline.txt in order to remove this.
So you can :

use a windows computer to add init=/bin/sh at the end of cmdline.txt in the /boot (fat) partition of the SD card use a linux computer (live distro if you don't have one), then mount root partition of the SD card and change ownership of sudoers.