As part of my switch to a new theme, I followed the instructions in the harbor theme to add the theme:
$ # install
$ mkdir themes
$ cd themes
$ git submodule add https://github.com/matsuyoshi30/harbor.git harbor
That worked and everything was rendering fine using hugo serve on my main system.
Sometimes, I use a different system to write posts and I wanted to get the updated code on there. After merging my pull request in github, I ran git pull to get the site updated on my laptop and created a new post.
Running hugo serve showed a problem: shortcode: template for shortcode "search" not found. This is a feature in the theme that was not working. What I had forgotten to do was init and update the submodule:
$ git submodule init
$ git submodule update
It all works now and I’m enjoying the new theme.