I wish we could switch to a dark theme. I'm not sure why it's dark in spotlight and light in other areas. I prefer dark. But there is no choice. You can download 3rd party keyboards that are dark. I use SwiftKey and TouchPal. Both have dark keyboards. I use SwiftKey the most. It has predictive and Swipe.
SwiftKey Keyboard by SwiftKey
https://appsto.re/us/qWsw2.i
TouchPal Keyboard - Theme, Swipe & Emoji by TouchPal HK Co., Ltd.
https://appsto.re/us/BQdo2.i
Sent from my ancient but trustworthy iPhone 5.