Fix missing 'THEME' key

Change-Id: If1a296c3c9bb1fd7de2effa107d898a482754ed8
Closes-bug: 1603307

horizon/themes.py

@@ -40,7 +40,10 @@ _local = threading.local()
 
 # Get the themes from settings
 def get_themes():
-    return getattr(settings, 'AVAILABLE_THEMES', [])
+    return getattr(settings, 'AVAILABLE_THEMES',
+                   [(get_default_theme(),
+                     get_default_theme(),
+                     os.path.join(get_theme_dir(), get_default_theme()))])
 
 
 # Get the themes dir from settings