@knappen Although this will work, I would strongly recommend against editing plugin files, in Jetpack or any other plugins. When you do so, it becomes more complicated to update the plugin without losing your changes.
If you want to make CSS changes to a plugin, it's usually best to just add your own CSS to your theme stylesheet, to a custom CSS file, or to the Custom CSS editor in Jetpack. This way, your changes won't get overwritten next time you update the plugin.