How can I change the existing css of shop frontend?
You can create theme to change design of shop system.
Second way is to override view of bagisto.
So, you can override master blade of shop package and change its css path with your css path.
please follow this one - https://forums.bagisto.com/topic/221/how-to-override-view-file-in-bagisto
I want to confirm one thing more, either i need to define new routes for custom theme or not?
You don't need to define new routes.