Instead of using fbcon (the default, in-kernel framebuffer-based terminal emulator), use fbterm (a userspace framebuffer terminal emulator, which supports backgrounds) with fbv as specified in the fbterm man page.
It looks like there’s also a patch, fbcondecor, to add background support to fbcon, but it’s not in vanilla kernel.
how would they change their wallpaper if they only use cli?
You make one from ascii!
Instead of using fbcon (the default, in-kernel framebuffer-based terminal emulator), use
fbterm(a userspace framebuffer terminal emulator, which supports backgrounds) withfbvas specified in thefbtermman page.It looks like there’s also a patch, fbcondecor, to add background support to fbcon, but it’s not in vanilla kernel.