chore(wiki): add a section about css usage, behavior and required B…

…abel preset (#8323)

wiki/consuming-eui/README.md

@@ -49,9 +49,15 @@ import { findTestSubject } from '@elastic/eui/lib/test'; // Enzyme
 import { findByTestSubject, render, screen } from '@elastic/eui/lib/test/rtl'; // React Testing Library
 ```
 
+### Custom styles
+
+EUI uses a CSS-in-JS approach for styling, specifically [Emotion](https://emotion.sh) library. We recommend using the `css` function (read more [here](https://emotion.sh/docs/css-prop#use-the-css-prop)) from `@emotion/react`, which automatically sets the generated `className` and combines all passed styles into a single ruleset. To enable `css` concatenation, you'll need the [Babel preset](https://www.npmjs.com/package/@emotion/babel-preset-css-prop).
+
+You can find more information regarding Emotion usage with EUI [here](https://github.com/elastic/eui/discussions/6828).
+
 ### Theming
 
-EUI uses CSS-in-JS (specifically [Emotion](https://emotion.sh)) for its styles and theming. As such, we require an `<EuiProvider>` wrapper around your application in order for various theme-related UI & UX (such as dark/light mode switching) to work as expected.
+EUI leverages [Emotion](https://emotion.sh) for its theming as well. As such, we require an `<EuiProvider>` wrapper around your application in order for various theme-related UI & UX (such as dark/light mode switching) to work as expected.
 
 ```jsx
 import React from 'react';