Removing parent theme CSS without editing parent theme

You have two options here

  • In a child theme, dequeue and deregister the scripts and style you are not going to need. Just remember to hook your function to wp_enqueue_scripts with a lower priority (higher number)

  • Use the twentyfifteen as a base to a new theme. I have successfully used bundled themes in the past to create new, selfstanding themes. That is one of the reasons the bundled themes are there for. The only thing you really need to change is the stylesheet header to something unique to avoid updates, and then make and break as you like.