I am setting up the base for an app with MUI v5 and TypeScript. I want to extend the MUI theme with some custom properties which add to the existing default properties.

My theme.ts config looks like that:

import { createTheme, ThemeOptions } from "@mui/material/styles";



interface IThemeOptions extends ThemeOptions {}



export const themeStyles = {

    palette: {...},

    breakpoints: {...},

    typography: {

        h1: {...},

        h2: {...},

        h3: {...},

        h4: {...},

        h5: {...}

        h6: {...},

        button: {...},

        body1: {...},

        body2: {...},

        //this is the custom prop I want to add

        h1Bold: {

            fontFamily: '"Roboto", "Helvetica", "Arial", sans-serif',

            fontWeight: 700,

            fontSize: "3.3125rem",

            lineHeight: "1.15em",

            color: "#1D1D1D",

            marginTop: "20px",

            marginBottom: "10px",

        },

    },

};



const theme = createTheme(themeStyles as IThemeOptions);



export default theme;

I call the "h1Bold" property in App.tsx like that:

import React from "react";



// Mui

import Box from "@mui/material/Box";

import Typography from "@mui/material/Typography";



interface IProps {}



const App: React.FC = (): JSX.Element => {

    return (

        <Box

            className="App"

            component="div"

            sx={{

                backgroundColor: "white",

            }}

        >

            <Typography variant="h1Bold">

                I am the App

            </Typography>

        </Box>

    );

};



export default App;

This gives me the Error:

    No overload matches this call.

  "Overload 1 of 2, '(props: { component: ElementType<any>; } & SystemProps<Theme> & { align?: "right" | "left" | "inherit" | "center" | "justify" | undefined; children?: ReactNode; ... 6 more ...; variantMapping?: Partial<...> | undefined; } & CommonProps & Omit<...>): Element', gave the following error.

    Type '"h1Bold"' is not assignable to type '"button" | "caption" | "h1" | "h2" | "h3" | "h4" | "h5" | "h6" | "inherit" | "subtitle1" | "subtitle2" | "body1" | "body2" | "overline" | undefined'.

  Overload 2 of 2, '(props: DefaultComponentProps<TypographyTypeMap<{}, "span">>): Element', gave the following error.

    Type '"h1Bold"' is not assignable to type '"button" | "caption" | "h1" | "h2" | "h3" | "h4" | "h5" | "h6" | "inherit" | "subtitle1" | "subtitle2" | "body1" | "body2" | "overline" | undefined'."

So I understand this error that I have to update the definition somehow. How can I do that so that TypeScript understands the newly added "h1Bold" property?

My index.ts looks like this:

import React from "react";

import ReactDOM from "react-dom";

import { BrowserRouter } from "react-router-dom";

import { Provider } from "react-redux";

import store from "./redux/store";

import { persistStore } from "redux-persist";

import { PersistGate } from "redux-persist/integration/react";

import { ThemeProvider } from "@mui/material/styles";

//this is the theme created with createTheme()

import theme from "./config/theme/theme";



import App from "./App";

import "./index.css";



// Redux-Persist

let persistor = persistStore(store);



ReactDOM.render(

    <React.StrictMode>

        <ThemeProvider theme={theme}>

            <Provider store={store}>

                <PersistGate loading={null} persistor={persistor}>

                    <BrowserRouter>

                        <App />

                    </BrowserRouter>

                </PersistGate>

            </Provider>

        </ThemeProvider>

    </React.StrictMode>,

    document.getElementById("root")

);

Would be great if someone could give me some insight or has a solution how to extend and read from the MUI theme more easily with having TypeScript in use.

Cheers