Copy this folder into Nebula's extensions directory to install:
Windows: %LOCALAPPDATA%\Nebula\User Data\extensions\hello-nebula\
macOS: ~/Library/Application Support/Nebula/User Data/extensions/hello-nebula/
Linux: ~/.local/share/Nebula/User Data/extensions/hello-nebula/
(or $XDG_DATA_HOME/Nebula/User Data/extensions/hello-nebula/)
Or open Settings > Plugins — the install path is shown there.
Then 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)