On the Android version of Flappy Bird, you can change the background from day to night, and from night to day. There’s still some uncertainty on how to do that, though, a recommended method would simply be to change the time from sometime during the day to sometime during the night on your phone or on your tablet. Unfortunately, you cannot change to night graphics on the iOS version yet.
However, if you want to change the graphics around on the iOS, go to the settings on your phone or iPad, then go to the accessibility area, and invert the colors. While inverted colors look ugly, switching back and forth between inverted colors and standard colors should aid concentration.
There are a few ways to get rid of/remove ads. You may have noticed that if you are playing the game in an area with bad reception, the ads won’t show up. Duplicate this by simply activating Airplane Mode or using another method to shut off your cellular data and/or Wi-Fi. However, I don’t recommend this since that’s how the developer makes his living. But it does help concentration.
You can also enable the Zoom function in the iOS if you so choose under the Accessibility area (I don’t know how to do it on Android). Use this to drag the ads clear off of the screen. Be aware that this will also make pipes show up faster on the screen, though, and require you to change up your timing.
Update the App Store on your phone to automatically update the apps, so that you don’t forget to upgrade Flappy Bird when the new upgrades come out, especially on iOS. Expect added functionality including night time mode added, as well as goodies such as making it slightly easier to play.