[EuiProvider] Fix SSR crashing during new system color mode detection

packages/eui/changelogs/upcoming/8040.md

@@ -0,0 +1,3 @@
+**Bug fixes**
+
+- Fixed `EuiProvider`'s system color mode detection causing errors during server-side rendering

packages/eui/src/components/provider/system_color_mode/system_color_mode_provider.server.test.tsx

@@ -0,0 +1,29 @@
+/**
+ * @jest-environment node
+ */
+/* eslint-disable local/require-license-header */
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0 and the Server Side Public License, v 1; you may not use this file except
+ * in compliance with, at your election, the Elastic License 2.0 or the Server
+ * Side Public License, v 1.
+ */
+
+import React from 'react';
+import { renderToString } from 'react-dom/server';
+
+import { EuiSystemColorModeProvider } from './system_color_mode_provider';
+
+describe('EuiSystemColorModeProvider', () => {
+  it('handles server-side rendering without crashing', () => {
+    const children = jest.fn(() => <>Test</>);
+
+    const renderOnServer = () =>
+      renderToString(<EuiSystemColorModeProvider children={children} />);
+    expect(renderOnServer).not.toThrow();
+
+    expect(renderOnServer()).toEqual('Test');
+    expect(children).toHaveBeenCalledWith('LIGHT');
+  });
+});

packages/eui/src/components/provider/system_color_mode/system_color_mode_provider.tsx

@@ -14,10 +14,13 @@ export const COLOR_MODE_MEDIA_QUERY = '(prefers-color-scheme: dark)';
 export const EuiSystemColorModeProvider: FunctionComponent<{
   children: (systemColorMode: EuiThemeColorModeStandard) => ReactElement;
 }> = ({ children }) => {
-  // Use optional chaining here for SSR or test environments
+  // Check typeof and use optional chaining for SSR or test environments
   const [systemColorMode, setSystemColorMode] =
     useState<EuiThemeColorModeStandard>(() =>
-      window?.matchMedia?.(COLOR_MODE_MEDIA_QUERY).matches ? 'DARK' : 'LIGHT'
+      typeof window !== 'undefined' &&
+      window.matchMedia?.(COLOR_MODE_MEDIA_QUERY)?.matches
+        ? 'DARK'
+        : 'LIGHT'
     );
 
   // Listen for system changes