Maybe for the offline.html you could take a look at chapter 9 from this documentation. Although, the documentation is for the index.html, maybe it will work the same for the login.html. In the documentation it's saying it's automatically generated when it's in the deployment folder, so like the index.html, you could try to copy the offline.html to your theme -> web folder and see if you can change it there and test if it works.
When you adjusted the Java action, did you press f4 in your Mendix project before running the app?