react-playground/node_modules/@mui/material/esm/styles/experimental_extendTheme.js

import extendTheme from "./createThemeWithVars.js";
let warnedOnce = false;
export default function deprecatedExtendTheme(...args) {
  if (!warnedOnce) {
    console.warn(['MUI: The `experimental_extendTheme` has been stabilized.', '', "You should use `import { extendTheme } from '@mui/material/styles'`"].join('\n'));
    warnedOnce = true;
  }
  return extendTheme(...args);
}