Вы можете просто использовать ThemeProvider из @ material-ui / core вместо MuiThemeProvider из @ material-ui / styles
import React from "react";
import ReactDOM from "react-dom";
import "./index.css";
import App from "./App";
import { createMuiTheme, ThemeProvider } from "@material-ui/core";
const theme = createMuiTheme({
palette: {
primary: {
main: "#ff8f00"
},
secondary: {
main: "#ffcc80"
}
}
});
ReactDOM.render(
<ThemeProvider theme={theme}>
<App />
</ThemeProvider>,
document.getElementById("root")
);