Color Keyboard for iOS 7 cydia tweak allows you to Colorize keyboard on you iPhone.
Now, you can change colors of key buttons or you can add background image on keyboard.
After installation, Settings > Color Keyboard > Select Theme > (Tap the Theme) > Save & Respring (Top Right).
[!]Important[!] Theme folder is changed: new folder is /Library/ColorKeyboard/Themes/
No new icons are added to your homescreen. You can configure options from your Settings app.