@mui/material/styles/adaptV4Theme.js

import { createBreakpoints, createSpacing } from '@mui/system';
export default function adaptV4Theme(inputTheme) {
  if (process.env.NODE_ENV !== 'production') {
    console.warn(['MUI: adaptV4Theme() is deprecated.', 'Follow the upgrade guide on https://mui.com/r/migration-v4#theme.'].join('\n'));
  }
  const {
    defaultProps = {},
    mixins = {},
    overrides = {},
    palette = {},
    props = {},
    styleOverrides = {},
    ...other
  } = inputTheme;
  const theme = {
    ...other,
    components: {}
  };

  // default props
  Object.keys(defaultProps).forEach(component => {
    const componentValue = theme.components[component] || {};
    componentValue.defaultProps = defaultProps[component];
    theme.components[component] = componentValue;
  });
  Object.keys(props).forEach(component => {
    const componentValue = theme.components[component] || {};
    componentValue.defaultProps = props[component];
    theme.components[component] = componentValue;
  });

  // CSS overrides
  Object.keys(styleOverrides).forEach(component => {
    const componentValue = theme.components[component] || {};
    componentValue.styleOverrides = styleOverrides[component];
    theme.components[component] = componentValue;
  });
  Object.keys(overrides).forEach(component => {
    const componentValue = theme.components[component] || {};
    componentValue.styleOverrides = overrides[component];
    theme.components[component] = componentValue;
  });

  // theme.spacing
  theme.spacing = createSpacing(inputTheme.spacing);

  // theme.mixins.gutters
  const breakpoints = createBreakpoints(inputTheme.breakpoints || {});
  const spacing = theme.spacing;
  theme.mixins = {
    gutters: (styles = {}) => {
      return {
        paddingLeft: spacing(2),
        paddingRight: spacing(2),
        ...styles,
        [breakpoints.up('sm')]: {
          paddingLeft: spacing(3),
          paddingRight: spacing(3),
          ...styles[breakpoints.up('sm')]
        }
      };
    },
    ...mixins
  };
  const {
    type: typeInput,
    mode: modeInput,
    ...paletteRest
  } = palette;
  const finalMode = modeInput || typeInput || 'light';
  theme.palette = {
    // theme.palette.text.hint
    text: {
      hint: finalMode === 'dark' ? 'rgba(255, 255, 255, 0.5)' : 'rgba(0, 0, 0, 0.38)'
    },
    mode: finalMode,
    type: finalMode,
    ...paletteRest
  };
  return theme;
}