docs: use let and const in theme script

Author: atesgoral

docs/_includes/head_custom.html

@@ -1,13 +1,13 @@
 <script>
 (function() {
-  var storageKey = "mcp-ruby-sdk-docs-theme";
-  var validPreferences = ["system", "light", "dark"];
-  var preferenceLabels = {
+  const storageKey = "mcp-ruby-sdk-docs-theme";
+  const validPreferences = ["system", "light", "dark"];
+  const preferenceLabels = {
     system: "System",
     light: "Light",
     dark: "Dark"
   };
-  var mediaQuery = window.matchMedia ?
+  const mediaQuery = window.matchMedia ?
     window.matchMedia("(prefers-color-scheme: dark)") :
     { matches: false };
 
@@ -17,7 +17,7 @@
 
   function getStoredPreference() {
     try {
-      var preference = window.localStorage.getItem(storageKey);
+      const preference = window.localStorage.getItem(storageKey);
       return isValidPreference(preference) ? preference : "system";
     } catch (error) {
       return "system";
@@ -41,7 +41,7 @@
   }
 
   function nextPreference(preference) {
-    var index = validPreferences.indexOf(preference);
+    const index = validPreferences.indexOf(preference);
     if (index === -1 || index === validPreferences.length - 1) {
       return validPreferences[0];
     }
@@ -50,17 +50,17 @@
   }
 
   function setThemeToggleLabel(preference) {
-    var buttons = document.querySelectorAll("[data-theme-toggle]");
-    var label = "Theme: " + preferenceLabels[preference];
-    for (var index = 0; index < buttons.length; index += 1) {
+    const buttons = document.querySelectorAll("[data-theme-toggle]");
+    const label = "Theme: " + preferenceLabels[preference];
+    for (let index = 0; index < buttons.length; index += 1) {
       buttons[index].setAttribute("aria-label", label);
       buttons[index].setAttribute("title", label);
     }
   }
 
   function applyPreference(preference) {
-    var safePreference = isValidPreference(preference) ? preference : "system";
-    var theme = resolvedTheme(safePreference);
+    const safePreference = isValidPreference(preference) ? preference : "system";
+    const theme = resolvedTheme(safePreference);
 
     if (window.jtd && typeof window.jtd.setTheme === "function") {
       window.jtd.setTheme(theme);
@@ -74,7 +74,7 @@
     apply: applyPreference,
     getPreference: getStoredPreference,
     save: function(preference) {
-      var safePreference = isValidPreference(preference) ? preference : "system";
+      const safePreference = isValidPreference(preference) ? preference : "system";
       savePreference(safePreference);
       applyPreference(safePreference);
     }
@@ -83,13 +83,13 @@
   applyPreference(getStoredPreference());
 
   function initThemeToggle() {
-    var buttons = document.querySelectorAll("[data-theme-toggle]");
+    const buttons = document.querySelectorAll("[data-theme-toggle]");
     if (buttons.length === 0) {
       return;
     }
 
     setThemeToggleLabel(getStoredPreference());
-    for (var index = 0; index < buttons.length; index += 1) {
+    for (let index = 0; index < buttons.length; index += 1) {
       buttons[index].addEventListener("click", function() {
         window.mcpDocsTheme.save(nextPreference(getStoredPreference()));
       });