revert to the default theme if the selected themes stylesheet is missing

deluge/ui/web/server.py

@@ -400,8 +400,11 @@ class TopLevel(resource.Resource):
         self.putChild("tracker", Tracker())
 
         theme = component.get("DelugeWeb").config["theme"]
+        if not os.path.isfile(rpath("css", "xtheme-%s.css" % theme)):
+            theme = CONFIG_DEFAULTS.get("theme")
         self.__stylesheets.insert(1, "/css/xtheme-%s.css" % theme)
 
+
     @property
     def scripts(self):
         return self.__scripts