Hi Anjitha,
I realize this was posted over 9 months ago, but I saw this show up on the GeoNet "More like this" sidebar as I was responding to another thread. I figure it might be worthwhile to have a response to this question documented.
Q1) can someone guide me how to change existing portal map viewer and add more functions, tools + customize portal templates as requires to my company? ... How can I change the look and feel of portal map viewer?
A1) Customization of the Portal map viewer is NOT supported. If you were to customize the Portal map viewer, this may cause issues when you attempt to upgrade your Portal instance to a later release. So while it is technically possible, we don't recommend that you do it, hence why there is no documentation for this.
Q2) Is there any sample templates I can download ? and how can I configure portal to use those downloaded templates?
A2) FYI, there are sample Web application templates - which are included with Portal for ArcGIS. Learn more here:
Configurable application templates—Portal for ArcGIS (10.3 and 10.3.1) | ArcGIS for Server
And the code for these application templates can be downloaded and customized. You would host these customized app templates on your own web server, then you can reference them back in Portal and used as custom templates.
Hope this helps,
