On advice from @phenomlab I uninstalled the plugin, rebuilt, installed plugin again, rebuilt and it was ok.
This had also happened to himself before.
I've created a custom theme and have added it to the node_modules of my nodeBB project. I've then added a line to .gitignore such that the theme is added to my git repository. I have then pushed the code to Heroku but can't get the theme to select.
I go to Admin > Appearance > Themes and there I can see my new theme. I click Select Theme then go to Dashboard and hit Restart to restart NodeBB as requested.
After this happens, I go back to Admin > Appearance > Themes but the default Persona theme is still set.
I've tried running a bash shell on Heroku but ./nodebb restart tells me that
"NodeBB could not be restarted, as a running instance could not be found."
This is likely to do with the way Heroku manages processes.
I saw this thread about the same issue but the OP seemed to have given up and moved to a different hosting solution: https://community.nodebb.org/topic/2063/created-a-new-theme-now-what/2
Or perhaps there is some way to specify which theme to use when starting the NodeBB server? I can't see a way to do this in the config.json file. Can it be changed on the command line? I'm starting it on Heroku by just running the app.js: