Move skin background to separate file

osu.Game/Graphics/Backgrounds/SkinBackground.cs

@@ -0,0 +1,25 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using osu.Framework.Allocation;
+using osu.Game.Skinning;
+
+namespace osu.Game.Graphics.Backgrounds
+{
+    internal class SkinBackground : Background
+    {
+        private readonly Skin skin;
+
+        public SkinBackground(Skin skin, string fallbackTextureName)
+            : base(fallbackTextureName)
+        {
+            this.skin = skin;
+        }
+
+        [BackgroundDependencyLoader]
+        private void load()
+        {
+            Sprite.Texture = skin.GetTexture("menu-background") ?? Sprite.Texture;
+        }
+    }
+}