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While there are number of jailbreak tweaks available to customize the look and feel of the Notification Center, Vexillum seems to be the first jailbreak tweak that allows you to customize banner notifications.
Vexillum is quite a simple jailbreak tweak. It allows you to change the color and font of banner notifications. Vexillum is developed by the same developer who recently released Lorem - a jailbreak tweak that allows you to clear notifications with just a swipe.