If you spend enough time online you might've heard about homebrew and custom firmware.
This isn’t a tutorial on how to hack your 3DS. For that, go to 3ds.hacks.guide.
Here you can find heaps of user-generated themes with new ones being made all the time. My current one is this miku one.

Back when online was still a thing, there was this app called Nintendo Badge Arcade where you could play virtual claw machines to earn decorative badges to put on your homescreen.
In hindsight, paying money to gamble for home screen stickers is pretty unethical. I wonder how many kids went broke trying to get some kirby png to put on their home menu.
I played it a lot when it was still around but now with cfw, hacking the badge system is pretty straightforward. Some homebrew let you install any image you'd like onto your 3DS badge collection and there are a bunch of premade collections on the internet. I'm using an Ace Attorney portraits collection to represent all of my folders, plus some kirby and miku ones.
