What you need, as RKurtzDmitriyev suggested, is to make the screen dark some other way. Luckily BIS has included a black cube object which avoids the need for skipping to night-time or finding someplace with no graphical details. See attached missionette.
Looking at your description.ext code I would suggest you get into the habit of terminating each line with a semicolon ; Also, give your dialog components unique idds - this will give you a lot more flexibility in how you access them with scripts.
Good luck with the project