The UI (javascript) is not part of firmware. The way this works is that these javascript files are stored on opensprinkler server, when you type in the IP address of the controller in the browser, the firmware returns code to the browser to reference the javascript code on opensprinkler server. And if you use the mobile app, these javascript files are embedded in the app.
Since you are going to customize the UI, what you should do is to download the UI code, unzip, and run the index.html in a browser. If you type in the IP of opensprinkler in a browser, or use the mobile app, those javascript codes are not your customized code so it won’t work.