Okay, to do the UI stuff..
Copy the packed.png file to /var/lib/hakchi/rootfs/usr/share/ui/snes-eur/resources/sprites/
Download this file and put it in /etc/preinit.d
Reboot the SNES Mini.
I think I know what my mistake was.
I just went with the eraser tool and erased all over the "game selection borders" (basically overwriting it with the color white). Instead I should have been very careful just to delete the actual border, right?