[Android] Implement material3 support for CheckBox

src/Core/src/Platform/Android/CheckBoxExtensions.cs

@@ -1,4 +1,5 @@
-using Android.Content.Res;
+using System.Runtime.CompilerServices;
+using Android.Content.Res;
 using Android.Graphics;
 using AndroidX.AppCompat.Widget;
 using AndroidX.Core.Widget;
@@ -9,6 +10,12 @@ namespace Microsoft.Maui.Platform
 {
 	public static class CheckBoxExtensions
 	{
+		// Store the original theme button tint per checkbox instance to support:
+		// - Per-Activity theming (different Activities can have different themes)
+		// - Theme switching at runtime (dark mode, Material2/3 toggle)
+		// - Thread safety (no shared mutable state)
+		static readonly ConditionalWeakTable<AppCompatCheckBox, ColorStateList> _defaultButtonTintCache = new();
+
 		public static void UpdateBackground(this AppCompatCheckBox platformCheckBox, ICheckBox check)
 		{
 			var paint = check.Background;
@@ -34,21 +41,39 @@ public static void UpdateForeground(this AppCompatCheckBox platformCheckBox, ICh
 
 		internal static ColorStateList GetColorStateList(this AppCompatCheckBox platformCheckBox, ICheckBox check)
 		{
-			AColor tintColor;
+			// Cache the original theme button tint for this checkbox instance before we modify it.
+			// This must happen before SetButtonTintList is called, as that will overwrite ButtonTintList.
+			if (!_defaultButtonTintCache.TryGetValue(platformCheckBox, out var defaultButtonTintList))
+			{
+				var currentTint = platformCheckBox.ButtonTintList;
+				if (currentTint is not null)
+				{
+					_defaultButtonTintCache.Add(platformCheckBox, currentTint);
+					defaultButtonTintList = currentTint;
+				}
+			}
 
 			// For the moment, we're only supporting solid color Paint for the Android Checkbox
 			if (check.Foreground is SolidPaint solid)
 			{
 				var color = solid.Color;
-				tintColor = color.ToPlatform();
+				AColor tintColor = color.ToPlatform();
+				return ColorStateListExtensions.CreateCheckBox(tintColor);
 			}
-			else
+
+			if (RuntimeFeature.IsMaterial3Enabled)
 			{
-				Graphics.Color accent = platformCheckBox.Context?.GetAccentColor() ?? Graphics.Color.FromArgb("#ff33b5e5");
-				tintColor = accent.ToPlatform();
+				// Material 3: Use the original theme's buttonTint
+				if (defaultButtonTintList is not null)
+				{
+					return defaultButtonTintList;
+				}
 			}
 
-			return ColorStateListExtensions.CreateCheckBox(tintColor);
+			// Material 2: Use accent color
+			Graphics.Color accent = platformCheckBox.Context?.GetAccentColor() ?? Graphics.Color.FromArgb("#ff33b5e5");
+			AColor tintColor2 = accent.ToPlatform();
+			return ColorStateListExtensions.CreateCheckBox(tintColor2);
 		}
 	}
-}
+}