I had done this same thing with system font tampering. I had to use the Offical ZTE Avid 4G update zip in order to fix my device so no worries, there is a cure! However, you will lose any information in which you had on the phone as this update will return your system back to stock, as how you bought it. So your contacts and apps must be re-added.
Now, what you must do!
1: Download -
http://www.zteusa.com/media/wysiwyg/ZTE-Avid/ZTE_Avid_4G_SD_Card_Upgrade_-_ZIP_-_304MB.zip
2. Place the ZTE_Avid_4G_SD_Card_Upgrade_-_ZIP_-_304MB.zip in the root directory of your sdcard. Try to use another phone in order to copy to your SDCard or an SDcard reader if you have one.
2. Boot into stock recovery, assuming that you are still stock recovery, otheriwse your custom recovery can do the trick as well.
3. From your recovery navigate using volume up/down and power key for 'ok' and make your way to flash update zip from SDcard.
Once completed, your system is new and will boot! best of luck.