change default theme to dark

frontend/src/App.jsx

@@ -548,7 +548,7 @@ function writeCookie(name, value) {
 }
 
 function normalizeThemePreference(value) {
-  return value === 'dark' ? 'dark' : 'light'
+  return value === 'light' ? 'light' : 'dark'
 }
 
 function storedThemePreference() {
@@ -561,7 +561,7 @@ function storedThemePreference() {
   try {
     return normalizeThemePreference(window.localStorage.getItem(themePreferenceKey))
   } catch {
-    return 'light'
+    return 'dark'
   }
 }