Hi Nishan, sadly, there are not very many best practices around using CSS with React yet. Facebook suggests using "React inline styles" as I have done in this lesson (which you can either have inside the file as I did in this lesson, or export/import from another file). Some people are using separate CSS or pre-processor files (like Sass). Its really up to you. I personally really like inline styles, especially for state-specific styling, as I feel it helps to keep the component more self contained; but every project and component is different, so I would suggest trying out a few different styles and seeing what you like :) You can always use plain old CSS files with classes and apply them to components with the "className" prop if you don't feel comfortable with inline styles.
Does that make sense? I hope this helps!