@mui/material/styles/createGetSelector.js

import excludeVariablesFromRoot from "./excludeVariablesFromRoot.js";
export default theme => (colorScheme, css) => {
  const root = theme.rootSelector || ':root';
  const selector = theme.colorSchemeSelector;
  let rule = selector;
  if (selector === 'class') {
    rule = '.%s';
  }
  if (selector === 'data') {
    rule = '[data-%s]';
  }
  if (selector?.startsWith('data-') && !selector.includes('%s')) {
    // 'data-mui-color-scheme' -> '[data-mui-color-scheme="%s"]'
    rule = `[${selector}="%s"]`;
  }
  if (theme.defaultColorScheme === colorScheme) {
    if (colorScheme === 'dark') {
      const excludedVariables = {};
      excludeVariablesFromRoot(theme.cssVarPrefix).forEach(cssVar => {
        excludedVariables[cssVar] = css[cssVar];
        delete css[cssVar];
      });
      if (rule === 'media') {
        return {
          [root]: css,
          [`@media (prefers-color-scheme: dark)`]: {
            [root]: excludedVariables
          }
        };
      }
      if (rule) {
        return {
          [rule.replace('%s', colorScheme)]: excludedVariables,
          [`${root}, ${rule.replace('%s', colorScheme)}`]: css
        };
      }
      return {
        [root]: {
          ...css,
          ...excludedVariables
        }
      };
    }
    if (rule && rule !== 'media') {
      return `${root}, ${rule.replace('%s', String(colorScheme))}`;
    }
  } else if (colorScheme) {
    if (rule === 'media') {
      return {
        [`@media (prefers-color-scheme: ${String(colorScheme)})`]: {
          [root]: css
        }
      };
    }
    if (rule) {
      return rule.replace('%s', String(colorScheme));
    }
  }
  return root;
};