Add MUI system theming snippet

docs/data/base/getting-started/quickstart/quickstart.md

@@ -6,19 +6,19 @@
 
 `@mui/base` is completely standalone – run one of the following commands to add Base UI to your React project:
 
-### npm
+### With npm
 
 ```sh
 npm install @mui/base
 ```
 
-### yarn
+### With yarn
 
 ```sh
 yarn add @mui/base
 ```
 
-### pnpm
+### With pnpm
 
 ```sh
 pnpm add @mui/base
@@ -146,7 +146,33 @@ Here's a complete demo of the same button styled with Tailwind instead:
 
 MUI System's core utility is a [`styled` function](/system/styled/) that is equivalent to emotion's or styled-components' styled(). Interpolations or arguments that are functions called by `styled` receive the `theme` from an upper `ThemeProvider`.
 
-Most of the demos and examples in the Base UI documentation are styled with MUI system in this way.
+```tsx
+import * as React from 'react';
+import { ThemeProvider } from '@emotion/react';
+import ButtonUnstyled from '@mui/base/ButtonUnstyled';
+
+const theme = {
+  colors: {
+    primary: 'green'
+  }
+}
+
+const GithubButton = styled(ButtonUnstyled)(
+  ({ theme }) => `
+    background-color: ${theme.colors.primary/* => 'green' */};
+  `,
+);
+
+render(
+  <ThemeProvider theme={theme}>
+    <GithubButton>
+      Create Repository
+    </GithubButton>
+  </ThemeProvider>
+)
+```
+
+Most of the demos and examples in the Base UI documentation are styled with MUI system in this way. You can inspect the `theme` object used on this site in your browser console, or explore the default structure [here](/material-ui/customization/default-theme/).
 
 #### Styling `ButtonUnstyled` with MUI System
 
@@ -200,7 +226,7 @@ export default function App() {
 
 #### Using the `sx` prop
 
-MUI System supports the [`sx` prop](/system/getting-started/the-sx-prop/), which provides a quick way to apply ad-hoc styles using theme-aware values directly to any component created with `styled`.
+MUI System supports the [`sx` prop](/system/getting-started/the-sx-prop/), which provides a quick way to apply ad-hoc styles using theme-aware values to any component created with `styled`.
 
 ```tsx
 const GithubButton = styled(ButtonUnstyled)(