Material ui themeing Typoography

Hey all I have been trying to add a default margin bottom to all my typography. Struggling I dont understand the documentation.

const theme = createMuiTheme({
  typography: {
    subtitle1: {
      fontSize: 12,
    },
    body1: {
      fontWeight: 500,
    },
    button: {
      fontStyle: 'italic',
    },
  },
});

I tried writing

const theme = createMuiTheme({
  h1: {
   marginBottom: "20px" 
  },
});

its not working :(