dark-mode: Make message deletion modal compatible.

This makes the message deletion modal compatible with dark mode by
making the background color dark.

templates/zerver/delete_message.html

@@ -1,4 +1,4 @@
-<div id="delete_message_modal" class="modal hide new-style" tabindex="-1" role="dialog" aria-labelledby="delete_message_modal_label" aria-hidden="true">
+<div id="delete_message_modal" class="modal modal-bg hide new-style" tabindex="-1" role="dialog" aria-labelledby="delete_message_modal_label" aria-hidden="true">
     <div class="modal-header">
         <button type="button" class="close" data-dismiss="modal" aria-label="{{ _('Close') }}"><span aria-hidden="true">&times;</span></button>
         <h3 id="delete_message_modal_label">{{ _("Delete message") }} </h3>