fix: add optional chaining for theme switch event listener

Add optional chaining in the event listener of the theme switch button in toggle-theme.js file. This
is to avoid occuring an error in the console when `light & dark theme mode` is disabled.

public/toggle-theme.js

@@ -37,7 +37,7 @@ window.onload = () => {
   reflectPreference();
 
   // now this script can find and listen for clicks on the control
-  document.querySelector("#theme-btn").addEventListener("click", () => {
+  document.querySelector("#theme-btn")?.addEventListener("click", () => {
     themeValue = themeValue === "light" ? "dark" : "light";
     setPreference();
   });