Add vanilla theme switcher

astro/src/utils/theme.ts

@@ -0,0 +1,21 @@
+export function switchTheme(themeClass: string): void {
+    const bodyElement = document.body;
+    const classesToRemove = ['light', 'dark', 'autonomic'];
+    
+    bodyElement.classList.remove(...classesToRemove);
+    bodyElement.classList.add(themeClass);
+}
+
+export function handleThemeChange(event: Event): void {
+    const selectElement = event.target as HTMLSelectElement;
+    const selectedTheme = selectElement.value;
+    
+    switchTheme(selectedTheme);
+}
+
+export function initializeThemeSelector(): void {
+    const themeSelector = document.getElementById('theme-selector');
+    if (themeSelector) {
+        themeSelector.addEventListener('change', handleThemeChange);
+    }
+}