apply colors to slack-notification bg-color on monoblue dark

src/client/styles/scss/theme/mono-blue.scss

@@ -202,4 +202,9 @@ html[dark] {
       @include three-stranded-button(lighten($primary, 30%), $primary, darken($primary, 10%), darken($primary, 20%));
     }
   }
+  .grw-slack-notification {
+    .form-control {
+      background: $themedark;
+    }
+  }
 }