Light Bright iOS 7 & 8 cydia tweak allows you to change brightnesss using volume keys.
Simply hold down the Home button as you click the up and down volume buttons to brighten and dim your screen!
Switch to maximum or minimum brightness by holding down the home button and toggling the mute switch!
You can change the brightness from the lock screen, from an app, or springboard!
No settings to adjust.
After installation just hold the home button and use the volume buttons to adjust brightness!