Thanks for sharing the example use-case. In general, I would recommend using the core styling features of the platform rather than relying on Custom CSS whenever possible. For example, you should be able to use the native Theme + Style features to customize the background color of dashboards and tiles.
Custom CSS is more of an ‘advanced’ feature designed with a technical audience in mind. @Bloodtick_Jones put together a brief primer on how to use Custom CSS which might be helpful if you find yourself looking to use Custom CSS if a Theme/Style feature doesn’t cover your need.