[Q] Dark UI on stock ROM, rooted Xperia Z Ultra topic

I recently moved from Windows 8 phone to Android with Sony Xperia Z Ultra. I really miss the Dark Black UI that was there on my windows phone.

Can someone suggest a way by which I can make the UI - Black for Dialler, Contacts, Settings, Calculator, Calendar and wherever possible on my Xperia Z Ultra.

My phone is rooted with stock ROM and I am not intending to install any other ROM.

Also, when I go to Personalisation > Xperia Themes > Choose a particular Theme. The option under Theme Elements as System Colors is greyed out and unselectable, if that is the place where I can change the colors ?