revert: html background for widget (#13981)

Reverts chatwoot/chatwoot#13955

app/javascript/widget/composables/useDarkMode.js

@@ -1,4 +1,4 @@
-import { computed, watchEffect } from 'vue';
+import { computed } from 'vue';
 import { useMapGetter } from 'dashboard/composables/store';
 
 const isDarkModeAuto = mode => mode === 'auto';
@@ -23,10 +23,6 @@ export function useDarkMode() {
     calculatePrefersDarkMode(darkMode.value, systemPreference.value)
   );
 
-  watchEffect(() => {
-    document.documentElement.classList.toggle('dark', prefersDarkMode.value);
-  });
-
   return {
     darkMode,
     prefersDarkMode,