night-mode: Fix stream creation loading indicator background color.

static/styles/night_mode.scss

@@ -184,6 +184,7 @@ on a dark background, and don't change the dark labels dark either. */
     }
 
     .overlay,
+    #stream_creation_form #stream_creating_indicator:not(:empty),
     .emoji-popover-emoji:not(.reacted):focus {
         background-color: hsla(212, 28%, 8%, 0.75);
     }