add OsuMarkdownSeparator

Reference
d56352aeef/resources/assets/less/bem/osu-md.less (L19-L21)

osu.Game.Tests/Visual/UserInterface/TestSceneOsuMarkdownContainer.cs

@@ -62,5 +62,18 @@ This is markdown code block.
 ```";
             });
         }
+
+        [Test]
+        public void TestSeparator()
+        {
+            AddStep("Add Seperator", () =>
+            {
+                markdownContainer.Text = @"Line above
+
+---
+
+Line below";
+            });
+        }
     }
 }

osu.Game/Graphics/Containers/Markdown/OsuMarkdownContainer.cs

@@ -30,6 +30,8 @@ namespace osu.Game.Graphics.Containers.Markdown
 
         protected override MarkdownFencedCodeBlock CreateFencedCodeBlock(FencedCodeBlock fencedCodeBlock) => new OsuMarkdownFencedCodeBlock(fencedCodeBlock);
 
+        protected override MarkdownSeparator CreateSeparator(ThematicBreakBlock thematicBlock) => new OsuMarkdownSeparator();
+
         protected override MarkdownPipeline CreateBuilder()
             => new MarkdownPipelineBuilder().UseAutoIdentifiers(AutoIdentifierOptions.GitHub)
                                             .UseEmojiAndSmiley()

osu.Game/Graphics/Containers/Markdown/OsuMarkdownSeparator.cs

@@ -0,0 +1,26 @@
+// 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.Framework.Graphics;
+using osu.Framework.Graphics.Containers.Markdown;
+using osu.Game.Overlays;
+
+namespace osu.Game.Graphics.Containers.Markdown
+{
+    public class OsuMarkdownSeparator : MarkdownSeparator
+    {
+        private Drawable separator;
+
+        [BackgroundDependencyLoader]
+        private void load(OverlayColourProvider colourProvider)
+        {
+            separator.Colour = colourProvider.Background3;
+        }
+
+        protected override Drawable CreateSeparator()
+        {
+            return separator = base.CreateSeparator();
+        }
+    }
+}