OK, maybe that was close to going overboard ;). But if you want people to be able to customize how Discourse looks and behaves, which helps with its spread and adoption, you should make it as easy as possible to do this.
What if I want to give more visibility to some elements, and less to others? This is an aesthetics decision which Discourse users should feel free to make. There are several threads here that highlight the diversity of themes and visual customizations.
Hunting for elements through nth-child stuff is complex and will eventually break when Discourse is updated. Make it easy to work Discourse into other/existing platforms, styles and brands.