Update .env.example

.env.example

@@ -56,6 +56,7 @@ MIX_API_SEARCH="${API_SEARCH}"
 
 ACTIVITYPUB_INBOX=false
 ACTIVITYPUB_SHAREDINBOX=false
+HORIZON_DARKMODE=true
 
 # Set these both "true" to enable federation.
 # You might need to also run: