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;
+        }
+    }
+}

osu.Game/Screens/Backgrounds/BackgroundScreenDefault.cs

@@ -140,22 +140,5 @@ namespace osu.Game.Screens.Backgrounds
                     return $@"Menu/menu-background-{currentDisplay % background_count + 1}";
             }
         }
-
-        private class SkinnedBackground : Background
-        {
-            private readonly Skin skin;
-
-            public SkinnedBackground(Skin skin, string fallbackTextureName)
-                : base(fallbackTextureName)
-            {
-                this.skin = skin;
-            }
-
-            [BackgroundDependencyLoader]
-            private void load()
-            {
-                Sprite.Texture = skin.GetTexture("menu-background") ?? Sprite.Texture;
-            }
-        }
     }
 }