✨ feat: listen for changes on OS theme to switch accordingly

As long as the visitor has not changed the default theme, the site will
change between dark and light themes matching the OS setting.

static/js/main.js

@@ -1,27 +1,31 @@
 const themeSwitcher = document.querySelector('.theme-switcher input');
-const currentTheme = localStorage.getItem('theme');
+let currentTheme = localStorage.getItem('theme');
+let userHasManuallyChangedTheme = currentTheme !== null;
+
+function setTheme(theme) {
+  document.documentElement.setAttribute('data-theme', theme);
+  themeSwitcher.checked = theme === 'dark';
+  localStorage.setItem('theme', theme);
+  currentTheme = theme;
+}
 
-// detect the user's preferred color scheme and activate it
 if (currentTheme) {
-    document.documentElement.setAttribute( 'data-theme', currentTheme);
-    themeSwitcher.checked = currentTheme === 'dark';
+  setTheme(currentTheme);
 } else {
   const isSystemDark = window.matchMedia('(prefers-color-scheme: dark)').matches;
-    document.documentElement.setAttribute( 'data-theme', isSystemDark? 'dark' : 'light');
-    themeSwitcher.checked = isSystemDark;
+  setTheme(isSystemDark ? 'dark' : 'light');
 }
 
-// switch between themes
 function switchTheme(e) {
-    if (e.target.checked) {
-        document.documentElement.setAttribute('data-theme', 'dark');
-        localStorage.setItem('theme', 'dark');
-    }
-    else {
-        document.documentElement.setAttribute('data-theme', 'light');
-        localStorage.setItem('theme', 'light');
-    }
+  const newTheme = e.target.checked ? 'dark' : 'light';
+  setTheme(newTheme);
+  userHasManuallyChangedTheme = true;
 }
 
-// event listener on checkbox change
 themeSwitcher.addEventListener('change', switchTheme, false);
+
+window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', (e) => {
+  if (!userHasManuallyChangedTheme) {
+    setTheme(e.matches ? 'dark' : 'light');
+  }
+});