Make the prefers-color-scheme regression test independent of the host machine's Windows theme

Render_DemoTestSample_MatchesBaselineImage_Media queries and color scheme was failing on CI
(and would fail on any dev machine whose theme differs from whatever machine captured the
baseline): WinFormsAdapter/WpfAdapter.SystemColorScheme reads the real Windows app theme via
the registry, so the sample's prefers-color-scheme card renders differently depending on the
host's actual theme setting, not on anything the code under test controls.

Add RAdapter.SystemColorSchemeOverride, an internal test-only seam both adapters consult before
falling back to the live registry read. The integration test suite pins it to Light before any
sample renders, so the render is deterministic regardless of the running machine's theme.
Re-approved the one baseline this affects.

Author: jhaygood86

Source/HtmlRenderer.WPF/Adapters/WpfAdapter.cs

@@ -140,6 +140,8 @@ public override RColorScheme SystemColorScheme
         {
             get
             {
+                if (SystemColorSchemeOverride.HasValue)
+                    return SystemColorSchemeOverride.Value;
                 if (!_colorScheme.HasValue)
                     _colorScheme = WindowsTheme.GetAppsColorScheme();
                 return _colorScheme.Value;

Source/HtmlRenderer.WinForms/Adapters/WinFormsAdapter.cs

@@ -120,6 +120,8 @@ public override RColorScheme SystemColorScheme
         {
             get
             {
+                if (SystemColorSchemeOverride.HasValue)
+                    return SystemColorSchemeOverride.Value;
                 if (!_colorScheme.HasValue)
                     _colorScheme = WindowsTheme.GetAppsColorScheme();
                 return _colorScheme.Value;

Source/HtmlRenderer/Adapters/RAdapter.cs

@@ -113,6 +113,16 @@ public virtual RColorScheme SystemColorScheme
             get { return RColorScheme.Light; }
         }
 
+        /// <summary>
+        /// Test-only seam: when set, an adapter that would otherwise report a live system theme (WinForms/
+        /// WPF) returns this value from <see cref="SystemColorScheme"/> instead of querying the OS. Image-
+        /// regression tests render against a fixed baseline and must not depend on the host machine's own
+        /// theme setting - without this, a CI runner whose theme differs from whatever machine captured the
+        /// baseline fails a <c>prefers-color-scheme</c> sample for reasons unrelated to any real rendering
+        /// regression. Production code must never set this.
+        /// </summary>
+        internal static RColorScheme? SystemColorSchemeOverride { get; set; }
+
         /// <summary>
         /// Raised when <see cref="SystemColorScheme"/> has changed, so anything rendered against it can
         /// re-evaluate its <c>prefers-color-scheme</c> rules and repaint. Never raised by an adapter

Source/HtmlRenderer/HtmlRenderer.csproj

@@ -25,6 +25,15 @@ For existing implementations see: HtmlRenderer.WinForms, HtmlRenderer.WPF and Ht
   <ItemGroup>
     <None Include="..\..\README.md" Pack="true" PackagePath="\" />
   </ItemGroup>
+  <ItemGroup>
+    <!-- Grants the WinForms/WPF adapters (which read SystemColorSchemeOverride from their own
+         SystemColorScheme override) and the image-regression test assembly (which sets it) access to
+         RAdapter's internal members. SystemColorSchemeOverride is a test-only seam that makes
+         prefers-color-scheme rendering deterministic regardless of the host machine's own theme setting. -->
+    <InternalsVisibleTo Include="HtmlRenderer.WinForms" />
+    <InternalsVisibleTo Include="HtmlRenderer.WPF" />
+    <InternalsVisibleTo Include="HtmlRenderer.IntegrationTest" />
+  </ItemGroup>
   <ItemGroup>
 	<PackageReference Include="System.Resources.Extensions" Version="10.0.10" />
 	<PackageReference Include="HtmlKit" Version="1.2.0" />

Source/Test/HtmlRenderer.IntegrationTest/Baselines/Media queries and color scheme.png

@@ -2,6 +2,7 @@
 using System.Globalization;
 using System.Reflection;
 using System.Text;
+using TheArtOfDev.HtmlRenderer.Adapters;
 using TheArtOfDev.HtmlRenderer.Core.Entities;
 using TheArtOfDev.HtmlRenderer.Demo.Common;
 using TheArtOfDev.HtmlRenderer.WinForms;
@@ -119,6 +120,12 @@ private static void EnsureSamplesLoaded()
             }
 
             SamplesLoader.Init("Regression", typeof(HtmlRender).Assembly.GetName().Version.ToString());
+
+            // Baselines are captured once on some machine and compared everywhere after; a sample driven
+            // by prefers-color-scheme must not depend on whichever machine happens to run the test having
+            // the same Windows theme as that one did. Pin it so every render sees the same reported scheme.
+            RAdapter.SystemColorSchemeOverride = RColorScheme.Light;
+
             _samplesLoaded = true;
         }
     }