Updated on August 20 / v1.2.2
In case you don't remember how you used resource packs in Alpha 0.15.0. It's easy, just go to options and click the button where it says: "Manage".
And then this screen comes out and you can deactivate this. That's all! ;D
Seriously, those default .json UI files and old UI design is made originally by Mojang, actually are modified and remade by me for entertainment purposes.
Just enjoy and good luck! ;D