cdn-frontend/src/themes/index.js

import defaultTheme from "./default";
import { createTheme } from "@material-ui/core/styles";

const overrides = {
  typography: {
    h1: {
      fontSize: "3rem"
    },
    h2: {
      fontSize: "2rem"
    },
    h3: {
      fontSize: "1.64rem"
    },
    h4: {
      fontSize: "1.5rem"
    },
    h5: {
      fontSize: "1.285rem"
    },
    h6: {
      fontSize: "1.142rem"
    }
  }
};

const themes = {
  default: createTheme({ ...defaultTheme, ...overrides })
};

export default themes;