Make sure you're editing the templates within the theme directory, and not within the build/public/templates directory
Try copying the files over from your server and ensuring that they are indeed edited.
As I've written above, I've created the new theme by duplicating the folder "nodebb-theme-persona" in nodebb/node_modules, then renaming the new folder. So that's where I'm editing 😄
I've also checked whether the upload was successful - it is, files are edited. Is it possible this has to do with some caching issue in the server? Or perhaps how I created this theme - not by installing it through the command npm?