To declare global variables in React, we can use the React context API.

For instance, we write

const themes = {
  light: {
    foreground: "#000000",
    background: "#eeeeee",
  },
  dark: {
    foreground: "#ffffff",
    background: "#222222",
  },
};
const ThemeContext = React.createContext(themes.light);

class App extends React.Component {
  render() {
    return (
      <ThemeContext.Provider value="dark">
        <Toolbar />
      </ThemeContext.Provider>
    );
  }
}

to call React.createContext to create the ThemeContext.

Then we wrap ThemeContext.Provider to let the Toolbar component access the context values.

Then we use the useContext hook to access the theme.light value by writing

function Toolbar() {
  const theme = useContext(ThemeContext);
  return (
    <button style={{ background: theme.background, color: theme.foreground }}>
      I am styled by theme context!
    </button>
  );
}```

We call `useContext` with `ThemeContext` to get the value of `ThemeContext` as the value of `theme`.