Fixed problem that global_theme setting was not respected

This fixes issue #880

Author: jbtronics

templates/base.html.twig

@@ -38,7 +38,7 @@
 
 
 
-        {% if not app.user.theme is defined %}
+        {% if not app.user.theme is defined or app.user.theme is null %}
             {% set theme = global_theme %}
         {% else %}
             {% set theme = app.user.theme %}