@pawan-patil You are able to modify the source code and use it on your own site. You are only limited if you are intending to redistribute the code. If you intend to, then you must also open-source your changed code.
If in doubt, you'll want to contact a lawyer to review the GPLv3 license that comes with NodeBB.
@baris It finally worked. Here is what I did that solved. (For some other poor soul facing similar issue)
Deactivated Custom pages
Rebuilt templates (Didn't work)
Removed Build folder and rebuilt (Still didn't work)
Changed the theme to some other except person and rebuilt
Activated Custom Pages
Changed theme to persona
Removed Build folder and rebuilt (Finally started seeing the damn tpl files)