Better colors for the "get ready" screen

The card itself is transparent, but it still has a size of 30% of the
viewport because the bar is based on the size of it's container.

The start bar and text can still be green and red, respectively, but the
light blue was completely out of place.

src/widgets.rs

@@ -180,9 +180,9 @@ fn spawn_get_ready(mut commands: Commands, mut timer: ResMut<ReadySetGoTimer>) {
             height: Val::Percent(30.),
             ..default()
         },
-        BackgroundColor(LIGHT_BLUE.into()),
+        BackgroundColor(Color::NONE),
         children![
-            (Text::new("Get Ready!"), TextColor(BLACK.into())),
+            (Text::new("Get Ready!"), TextColor(WHITE.into())),
             (
                 CountdownBar,
                 Node {