Night theme not coming in iBooks of my iPad air 2

Hi,

I recently purchased iPad air 2 and it is with iOS 8.1. But still I can't find the night theme when I opened my PDFs file in iBook. Can you please suggest what I need to do to make this feature available. The fact is the aA icon on the top right from where I should get that feature is not showing at all.

Thanks,

Mainak