New source found from dndbeyond.com

ddb_main/node_modules/@mui/system/esm/GlobalStyles/GlobalStyles.js

@@ -2,16 +2,40 @@
 
 import * as React from 'react';
 import PropTypes from 'prop-types';
-import { GlobalStyles as MuiGlobalStyles } from '@mui/styled-engine';
+import { GlobalStyles as MuiGlobalStyles, internal_serializeStyles as serializeStyles } from '@mui/styled-engine';
 import useTheme from '../useTheme';
 import { jsx as _jsx } from "react/jsx-runtime";
+function wrapGlobalLayer(styles) {
+  const serialized = serializeStyles(styles);
+  if (styles !== serialized && serialized.styles) {
+    if (!serialized.styles.match(/^@layer\s+[^{]*$/)) {
+      // If the styles are not already wrapped in a layer, wrap them in a global layer.
+      serialized.styles = `@layer global{${serialized.styles}}`;
+    }
+    return serialized;
+  }
+  return styles;
+}
 function GlobalStyles({
   styles,
   themeId,
   defaultTheme = {}
 }) {
   const upperTheme = useTheme(defaultTheme);
-  const globalStyles = typeof styles === 'function' ? styles(themeId ? upperTheme[themeId] || upperTheme : upperTheme) : styles;
+  const resolvedTheme = themeId ? upperTheme[themeId] || upperTheme : upperTheme;
+  let globalStyles = typeof styles === 'function' ? styles(resolvedTheme) : styles;
+  if (resolvedTheme.modularCssLayers) {
+    if (Array.isArray(globalStyles)) {
+      globalStyles = globalStyles.map(styleArg => {
+        if (typeof styleArg === 'function') {
+          return wrapGlobalLayer(styleArg(resolvedTheme));
+        }
+        return wrapGlobalLayer(styleArg);
+      });
+    } else {
+      globalStyles = wrapGlobalLayer(globalStyles);
+    }
+  }
   return /*#__PURE__*/_jsx(MuiGlobalStyles, {
     styles: globalStyles
   });