Fix crash when viewing settings in osm_viewer

Longer term I think we should default to day_theme for all widgetry apps.
That will be easier if we can get rid of pregame theme all together, but
that requires re-styling the pre-game screens (which we want to do, but
that will take time).

map_gui/src/colors.rs

@@ -40,6 +40,7 @@ impl ColorSchemeChoice {
         vec![
             Choice::new("day mode", ColorSchemeChoice::DayMode),
             Choice::new("night mode", ColorSchemeChoice::NightMode),
+            Choice::new("pregame", ColorSchemeChoice::Pregame),
             Choice::new("sam green day", ColorSchemeChoice::SAMGreenDay),
             Choice::new("sam desert day", ColorSchemeChoice::SAMDesertDay),
             Choice::new("bap", ColorSchemeChoice::BAP),