Can I Mod an Android App

First, let me clarify a few terms so the next paragraph makes sense:App: A software application designed to run on a smartphone or tablet.Mod: An app for a Smartphone that modifies another app (either by copying the existing app or modifying it directly)Target: The app to be modified.I think I found a way to mod apps on the Android phone. I want to mod the resources (text, graphics, sounds, colors, etc) of an app (possibly for a small fee). The mod would require the target app already be installed. The mod would then copy the app and modify it dynamically on their phone. I do not want to steal credit for the target app, just alter the user experience. Can I release this “mod” app without being in violation of copyright law?If not, how can I get a “license” from the app developer?Can I copyright my mod?Which legal codes should I read?