Style in react

Regarding to styling in react chapter, I know that we can put the style in a separate CSS file, in this case, how can we assign CSS style to the letterStyle variable?

There are a number of ways to style using CSS in React. Depending which one you work with will determine how and where you write and access your CSS variables, and whether they will be locally or globally scoped to your app’s or component’s CSS. The most basic is to import a index.css file that contains your global css variables. To make them global to the whole project you can use :root as your selector.