it's not secret, it's a glitch if you keep scrolling on the activity feed for a long time then the background will turn into a dark blue color and all of the images will not load

What if I do it on a New 3DS XL?

its a glitch my bro. like the other guy said if you scroll for to long that will happen. its because the old 3ds can only handle so much

the theme is very good looking and not at all hideous