Zettlr 2.0 introduced the new assets manager which allows you to manage all static assets that are stored as files in your user data directory. Among these are the new defaults files, your custom CSS, and snippets. While it is possible to edit these files externally, the assets manager gives you a comprehensive means to do so from within the app itself.
You can access the assets manager via "Zettlr -> Assets Manager" (macOS) or "File -> Preferences -> Assets Manager" (Windows/Linux).
The assets manager allows you to edit your files using correct syntax highlighting and makes it easier than external editors. To see what you can edit and how to do so productively, see the corresponding parts of the documentation: