♻️ refactor: use `const` in theme initialization

Refactor the theme initialization script to use 'const' instead of 'let'
for the 'currentTheme' variable, as the value is not expected to change
after initialization.

This makes the code more clear and prevents accidental
re-assignment.

static/js/initialize_theme.js

@@ -1,5 +1,5 @@
 (function () {
-    let currentTheme = localStorage.getItem('theme');
+    const currentTheme = localStorage.getItem('theme');
     if (currentTheme) {
         document.documentElement.setAttribute('data-theme', currentTheme);
     } else {

static/js/initialize_theme_min.js

@@ -1 +1 @@
-!function(){let e=localStorage.getItem("theme");if(e)document.documentElement.setAttribute("data-theme",e);else{let t=window.matchMedia("(prefers-color-scheme: dark)").matches;document.documentElement.setAttribute("data-theme",t?"dark":"light")}}();
+!function(){const e=localStorage.getItem("theme");if(e)document.documentElement.setAttribute("data-theme",e);else{const t=window.matchMedia("(prefers-color-scheme: dark)").matches;document.documentElement.setAttribute("data-theme",t?"dark":"light")}}();