Yes, you would need to create a background process for that. Using regular homebrew would be a problem because that every time the system reboots the process would close (i think). So i think it's more possible on A9LH or NTR.
It would be nice if a saturation boost were possible too, the 3DS screen is too washed out. :p But i think it would be even more complicated to do. :/
Btw, isn't it possible to just apply a post processing filter using just the GPU through a process or something? (i'm not specialist or...
You can extract the background from the PSD file that i posted. Not only that, but also customize it the way you want.
https://gbatemp.net/threads/menuhax-2-0-custom-main-screen-images.401528/page-25#post-5998937
It's not the movies or games downloads that I would worry about, like breaking into networks, downloading encrypted things, spying on network traffic. I have seen so many "Top Secret" seals on files when I was a kid
I was obsessed with finding UFOs, a surprising amount of US files where stashed on computers in other countries, China back in the early 90s omg sooo much