How to create Service portal, Service portal pages and configure theme? What are pages and widgets? 

You probably waiting for coding part. Keep patience we will explore
once you will have good knowledge about Service Portal.
What are pages?
A portal page houses containers, rows, and columns which then contain widgets. By designing the layout of the page, and the widgets within it, developers construct the desired user experience. Pages are reusable across portals.
What are widgets?
Widgets are reusable components that make up the functionality of a portal page. Widgets define what a portal does and what information a user sees.