The latest version of iBooks has a “night” theme that is meant to make reading on the ipad in dim light less of a strain on your eyes. It’s activated via the fonts menu in iBooks, and simply toggles the default white-on-black color scheme to a black background with white text.

There is a tweak that lets you do the same toggle trick with any app, according to the Labnol site. Tap the Settings icon and choose General –> Accessibility –> Triple-click Home and and set it to “Toggle White on Black.” Press the Home button to exit Settings and launch any reading app. Triple-click the Home button on your iOS device in quick succession and it should enable white-on-black. Triple-click again to switch back to black=on-white.

How to Enable Night-Reading for any iPad or iPhone App.