Hey guys! I thought this would be helpful since when you walk in the store, the lady at the front walks up to you, and says you are not stylist enough. So I came up with a guide!
Note: You need to have the FULL map of Lumiose City, in order to do this. meaning you have to fix the power after the fourth gym.
1)Battle the One Star(in the lower half) and Two Star hotels(in an Alley way).
2)Go to a few cafe's and talk to everybody
3)Go to the Hotel and work for them for all of the games(Room Service, Making Bed, and Lost & Found)
4)Talk to the Viola's, the first gym leader's, sister in a store. (Her store has a sign with a book on it) Also talk with everyone in there.
5)Get the Contact/Lens case and shoot a film in the PR/TR Video. You can get the Contact/Lens Case from a girl and there.
Note: wear one, just to be safe.
6)Find/Defeat all of the trainers in the hidden alley ways.
After all of that stuff, you can go over there and the lady will say "Wow you're stylish! Come on in!" and you're welcomed inside.
Mens are at the top floor and Womens at the bottom floor.
Note: They are REALLY good looking clothes, but almost range from 80,000-120,000 Pokedollars for each item.
ENJOY!