Merge pull request #422 from edx/jarv/fix-theme-name

fixing default for THEME_NAME

playbooks/roles/edxapp/defaults/main.yml

@@ -392,10 +392,11 @@ worker_core_mult:
   lms_preview: 2
   cms: 2
 
-#Theming
-#To turn off theming, specify edxapp_theme_name: ''
-#Stanford, for example, uses edxapp_theme_name: 'stanford'
-edxapp_theme_name: ''
+# Theming
+# To turn off theming, specify edxapp_theme_name: !!null
+# Stanford, for example, uses edxapp_theme_name: 'stanford'
+#
+edxapp_theme_name: !!null
 edxapp_theme_source_repo: 'https://{{ COMMON_GIT_MIRROR }}/Stanford-Online/edx-theme.git'
 edxapp_theme_version: 'HEAD'