Inspired by a site’s layout? Recreate it from scratch with your own code — that’s learning. Copy-pasting their CSS is theft.
You can save individual web pages via Ctrl+S (or Cmd+S) for offline reading. This is generally allowed under fair use for personal reference. Kristina Fey Full Site Rip
If you are Kristina Fey (or any creator) and discover someone has performed a full site rip of your work: Inspired by a site’s layout