Material-UI v4 Menu (override styles of the dropdown component ul & li GLOBALLY)

mainApplicationComponent.jsx

'use strict';

import React from "react";
import { ThemeProvider } from '@material-ui/styles';
import CssBaseline from '@material-ui/core/CssBaseline';
import theme from 'theme.js';

export default class App extends React.Component {

    render() {
        return (
            <ThemeProvider theme={theme}>
                <CssBaseline />
                    {/* Your app content */}
            </ThemeProvider>
        );
    }
}

theme.js

'use strict';

import { createMuiTheme } from '@material-ui/core/styles';

const theme = createMuiTheme({
    overrides: {
        // Applied to the <ul> element
        MuiMenu: {
            list: {
                backgroundColor: "#cccccc",
            },
        },
        // Applied to the <li> elements
        MuiMenuItem: {
            root: {
                fontSize: 12,
            },
        },
    },
});

export default theme;