I was able to setup CKAN (Open Data Portal) similar to https://demo.ckan.org/. Now i want to customized how the portal looks. If you go through https://ckan.org/showcase you can see a lot of CKAN based data portal with excellent user interface / website design.
How can I customize how portal looks, is there any extension to do that ?
You can modify or extend the CKAN functionality by creating your own CKAN extension. Check out the official docs for writing extensions. To modify the default design you can extend the CKANs Jinja templates by following these docs