[Anthem]

Are you talking about the theme of the apps? The background changing to White from a dark setting might be the Auto-Night Theme setting unless you just mean that you don't like Apple's aesthetic decisions. If you mean when reading then when you are in iBooks touch the middle of the screen to pull up the reading options, touch the aA button and see if "Auto-Night Theme" is green (slid to on). If it is and you don't want it to be, just touch the slider and it will turn grey, now you can control the settings manually.

If it is really terrible you might try doing a reset of your Mini in Settings> General> Reset> Erase All Content and Settings. But think of this as a nuclear option that you don't necessarily want to do unless everything seems hopeless. And make sure to backup your iPad using iCloud or iTunes on your computer first.

If this is the first Mini to be released it is (internally) essentially an ancient iPad 2 and iOS 8.1 doesn't run super great on that hardware FOR NOW... there will be many improvements that will hopefully pump up the performance on those old devices.

My iPad Mini 2 hasn't had any problems with iOS 8 so far, but I did the update from a clean state.