[system][InitColorSchemeScript] Use let/const instead of var (#43468)

Signed-off-by: Siriwat K <siriwatkunaporn@gmail.com>
Co-authored-by: Siriwat K <siriwatkunaporn@gmail.com>
Co-authored-by: ZeeshanTamboli <zeeshan.tamboli@gmail.com>

Author: 3 people

packages/mui-system/src/InitColorSchemeScript/InitColorSchemeScript.tsx

@@ -92,13 +92,13 @@ export default function InitColorSchemeScript(options?: InitColorSchemeScriptPro
       dangerouslySetInnerHTML={{
         __html: `(function() {
 try {
-  var mode = localStorage.getItem('${modeStorageKey}') || 'system';
-  var colorScheme = '';
-  var dark = localStorage.getItem('${colorSchemeStorageKey}-dark') || '${defaultDarkColorScheme}';
-  var light = localStorage.getItem('${colorSchemeStorageKey}-light') || '${defaultLightColorScheme}';
+  let colorScheme = '';
+  const mode = localStorage.getItem('${modeStorageKey}') || 'system';
+  const dark = localStorage.getItem('${colorSchemeStorageKey}-dark') || '${defaultDarkColorScheme}';
+  const light = localStorage.getItem('${colorSchemeStorageKey}-light') || '${defaultLightColorScheme}';
   if (mode === 'system') {
     // handle system mode
-    var mql = window.matchMedia('(prefers-color-scheme: dark)');
+    const mql = window.matchMedia('(prefers-color-scheme: dark)');
     if (mql.matches) {
       colorScheme = dark
     } else {