Well, yea, Miiir shops are ALWAYS dark...
Lets do a little experiment, shall we?
Compare color depth:
http://www.zagadka.net/rubika/colors.jpg
As you can see from the above composite, clothing is brighter outside the Miiir shop at NIGHT than it is inside, even though the surroundings are better. I think someone f*ed up on the global lighting variable inside the shops... even the booths are darker. In short, everything not pre-rendered. Granted, the pic inside the shop is a tad small (my error), but that is pretty much what you see, even close up. YOu can see that it is much brighter inside the Happy Rebel. Or maybe that is just Cosmik's dancing spreading supernatural light.