Parallax-scroll the background by Y.

Shared/Camera.cs

@@ -11,6 +11,8 @@ namespace SemiColinGames {
     public int Height { get => bbox.Height; }
     public int Left { get => bbox.Left; }
     public int Top { get => bbox.Top; }
+    public int Bottom { get => bbox.Bottom; }
+
     public Point HalfSize { get => new Point(Width / 2, Height / 2); }
 
     private readonly Random random = new Random();

Shared/Scene.cs

@@ -74,13 +74,14 @@ namespace SemiColinGames {
       spriteBatch.Begin(SpriteSortMode.Deferred, null, SamplerState.LinearWrap, null, null);
       Rectangle bgTarget = new Rectangle(0, 0, camera.Width, camera.Height);
 
-      float xScale = 1f / 16;
+      float bgScale = 1f / 16;
       for (int i = 0; i < Textures.Backgrounds.Length; i++) {
-        int yOffset = Textures.Backgrounds[i].Get.Height - camera.Height - 24;
+        float yDiff = (world.Height - camera.Bottom) * bgScale;
+        float yOffset = Textures.Backgrounds[i].Get.Height - camera.Height - yDiff;
         Rectangle bgSource = new Rectangle(
-          (int) (camera.Left * xScale), yOffset, camera.Width, camera.Height);
+          (int) (camera.Left * bgScale), (int) yOffset, camera.Width, camera.Height);
         spriteBatch.Draw(Textures.Backgrounds[i].Get, bgTarget, bgSource, Color.White);
-        xScale *= 2;
+        bgScale *= 2;
       }
       spriteBatch.End();