Any changes you make to the plugin’s stylesheet will be overwritten when you update the plugin. Likewise with an edit to the theme’s stylesheet. Even if you create your own stylesheet and enqueue it via functions.php you will have redo the enqueueing when you update. Of course, if this is a theme (or child theme) of your own creation that isn’t a problem.
If you aren’t maintaining your own theme, or child theme, there isn’t a great way to do this, in my opinion. It would be nice if there were a global “user-functions.php” file. The closest thing to that that I can think of is a “must use plugin”, and I guess that is probably what I’d recommend.
- Create a directory called
mu-pluginsinwp-content - Create a file in that directory and add your functions to it.
See the Codex page for details about Must-Use Plugins. They do not behave exactly like ordinary plugins.