External CSS libraries can also be used to do CSS styling along with any other methods we discussed.
Most popular external CSS frameworks or libraries are,
Other ways to style your applications are using other React component libraries. Again, that alone won't be enough. But for simpler use cases, you can always use exiting component libraries.
Some of the popular ones are,
There are many more external libraries. You can check out different ones based on your use cases. Mostly, check for flexibility and maintainability of those libraries in long term. Some of them might be easier to start but hard to customize later on.