feat: Support Dark mode for the widget (#4137)

Co-authored-by: Pranav Raj S <pranav@chatwoot.com>

app/javascript/widget/mixins/darkModeMixin.js

@@ -0,0 +1,15 @@
+import { mapGetters } from 'vuex';
+
+export default {
+  computed: {
+    ...mapGetters({ darkMode: 'appConfig/darkMode' }),
+  },
+  methods: {
+    $dm(light, dark) {
+      if (this.darkMode === 'light') {
+        return light;
+      }
+      return light + ' ' + dark;
+    },
+  },
+};