@Shashank_rajak Dev preview will NOT include the lock screen theming app. That will come later. I DO NOT KNOW the date. 🙂
— joebelfiore (@joebelfiore) April 12, 2014
Joe Belfiore today confirmed on Twitter that Lockscreen theming feature that was demoed at BUILD keynote is now included in Windows Phone 8.1 developer preview. However, you can expect this feature during the final update that will arrive in the coming months.