Copy this folder into Nebula's extensions directory to install:

  %LOCALAPPDATA%\Nebula\User Data\extensions\hello-nebula\

Then open Settings > Plugins and click Reload Plugins (or restart Nebula).

manifest.json fields:
  id, name, version, description, enabled
  content_scripts[].matches  (e.g. "*://*/*", "*://*.youtube.com/*", "<all_urls>")
  content_scripts[].js / css (paths relative to this folder)