One step toward the removal of @material-ui/styles in v6.

Breaking changes

• The styled JSS utility is no longer exported from @material-ui/core/styles. You can use @material-ui/styles/styled instead. Make sure to add a ThemeProvider at the root of your application, as the defaultTheme is no longer available. If you are using this utility together with @material-ui/core, it's recommended you use the ThemeProvider component from @material-ui/core/styles instead.

  -import { styled } from '@material-ui/core/styles';
  +import { styled } from '@material-ui/styles';
  +import { createTheme, ThemeProvider } from '@material-ui/core/styles';
  
  +const theme = createTheme();
   const MyComponent = styled('div')(({ theme }) => ({ background: theme.palette.primary.main }));
   
   function App(props) {
  -  return <MyComponent />;
  +  return <ThemeProvider theme={theme}><MyComponent {...props} /></ThemeProvider>;
   }

• I have followed (at least) the PR section of the contributing guide.