[#1] Add default theme env variable

docker-compose.yml

@@ -13,6 +13,7 @@ services:
       - FIRST_NAME=first name                               # Your first name
       - LAST_NAME=lastname                                  # Your last name
       - COUNTRY=US                                          # ISO-3166-1 alpha-2 country code, https://en.wikipedia.org/wiki/ISO_3166-1#Codes
+      - DEFAULT_THEME=light                                 # 'light' or 'dark'
       - SMTP_HOST=smtp.example.com
       - SMTP_PORT=465
       - SMTP_SECURE=true