How to edit an existing custom theme locally?


I’m new to Ghost and am looking to make edits to an existing theme locally. I have downloaded the theme zip file from site design page. I have then made a new directory locally and gone through the following steps in my terminal:

npm install ghost-cli@latest -g

ghost install local

http://localhost:2368/ is working after this process with the default casper theme, but I’m not sure how to migrate the downloaded existing theme into and have it working viewable on the local host. Can’t find any documentation on this.

Any help would be greatly appreciated.