Pull THEME_NAME from envs tokens if it's there.

If the FEATURES['USE_CUSTOM_THEME'] flag is set but the
'THEME_NAME' is not set.  We get a compilation error during
cms gather assets.

cms/envs/aws.py

@@ -150,6 +150,9 @@ TECH_SUPPORT_EMAIL = ENV_TOKENS.get('TECH_SUPPORT_EMAIL', TECH_SUPPORT_EMAIL)
 
 COURSES_WITH_UNSAFE_CODE = ENV_TOKENS.get("COURSES_WITH_UNSAFE_CODE", [])
 
+# Theme overrides
+THEME_NAME = ENV_TOKENS.get('THEME_NAME', None)
+
 #Timezone overrides
 TIME_ZONE = ENV_TOKENS.get('TIME_ZONE', TIME_ZONE)