Adds optional setting to enable devstack theming

The mako engine isn't able to find themed templates in devstack because
the path it checks is set before we enable comprehensive theming. This
Adds a settings section to  comment out if you want to enable
theming in devstack.

lms/envs/devstack_docker.py

@@ -86,3 +86,22 @@ SYSTEM_WIDE_ROLE_CLASSES.extend(['system_wide_roles.SystemWideRoleAssignment'])
 
 if FEATURES['ENABLE_ENTERPRISE_INTEGRATION']:
     SYSTEM_WIDE_ROLE_CLASSES.extend(['enterprise.SystemWideEnterpriseUserRoleAssignment'])
+
+########################## THEMING  #######################
+# If you want to enable theming in devstack, uncomment this section and add any relevant
+# theme directories to COMPREHENSIVE_THEME_DIRS
+
+# We have to import the private method here because production.py calls
+# derive_settings('lms.envs.production') which runs _make_mako_template_dirs with
+# the settings from production, which doesn't include these theming settings. Thus,
+# the templating engine is unable to find the themed templates because they don't exist
+# in it's path. Re-calling derive_settings doesn't work because the settings was already
+# changed from a function to a list, and it can't be derived again.
+
+# from .common import _make_mako_template_dirs
+# ENABLE_COMPREHENSIVE_THEMING = True
+# COMPREHENSIVE_THEME_DIRS = [
+#     "/edx/app/edxapp/edx-platform/themes/"
+# ]
+# TEMPLATES[1]["DIRS"] = _make_mako_template_dirs
+# derive_settings(__name__)