Hope this helps!
Hi Akhil Gupta
Copy index-example.html file from deployment\web folder within the app dire.
Paste it into theme\web folder
Rename it into an index.html file, and open the index.html file add anything you want.
Example: add <script src="alert.js"></script> and in theme\web folder create a file alert.js and in alert.js file create write alert(); and start your app.
It should work.