Evaluate @media against the platform, via two new adapter properties

@media conditions were never really evaluated: DomParser cascaded against a
hard-coded media type of "all" and CssData compared only the medium's type, so
every `@media screen` / `@media print` block was dropped, a feature-only query
like `@media (prefers-color-scheme: dark)` was dropped, and
`@media all and (prefers-color-scheme: dark)` matched unconditionally - applying
rules guarded by a condition nothing had tested.

RAdapter gains two virtuals describing the surface being rendered onto:

- DefaultMediaType - "screen" by default, overridden to "print" in
  PdfSharpAdapter
- SystemColorScheme - Light by default; WinForms and WPF read the Windows app
  theme (HKCU\...\Themes\Personalize\AppsUseLightTheme), cached and invalidated
  on SystemEvents.UserPreferenceChanged rather than read per query

MediaQueryContext captures that device - media type, viewport size in CSS px,
resolution and colour scheme - and MediaQueryMatcher evaluates a rule's
enclosing @media chain against it: width/height, orientation, aspect-ratio,
resolution, colour depth, and the prefers-* set. Its central policy is Media
Queries 4's "unknown feature is false": a feature the matcher cannot evaluate
drops its whole block rather than letting the rules inside apply untested. A
feature it does model but whose context is missing (a width query before the
viewport is known) stays permissive - that is an unknown context, not an
unsupported feature.

This is a deliberate behaviour change: @media screen blocks now apply on
WinForms/WPF and @media print blocks on PdfSharp, where previously neither did.

Also fixes colour resolution inside a function: the render-time tokenizer ran
outside value mode, where a digit-leading hex splits into a "#" delimiter plus a
dimension, so color-mix(in srgb, #ffffff 0%, #0000ff) failed to resolve while
the same value with named operands worked.

TestSamples/23 exercises all of it in the WinForms and WPF demos: a card that
follows the OS theme, one card per media type, a card proving an unevaluable
feature does not apply, and a viewport-width probe.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: 2 people

Source/Demo/Common/HtmlRenderer.Demo.Common.csproj

@@ -66,6 +66,7 @@
     <EmbeddedResource Include="TestSamples\20.Fonts decorations.htm" />
     <EmbeddedResource Include="TestSamples\21.Bullets.htm" />
     <EmbeddedResource Include="TestSamples\22.RTL.htm" />
+    <EmbeddedResource Include="TestSamples\23.Media queries and color scheme.htm" />
     <EmbeddedResource Include="TestSamples\30.Misc.htm" />
     <EmbeddedResource Include="TestSamples\31.ACID 1.htm" />
     <EmbeddedResource Include="TestSamples\32.Image in css content.htm" />

Source/Demo/Common/TestSamples/23.Media queries and color scheme.htm

@@ -0,0 +1,99 @@
+<html>
+<head>
+<style>
+    body { font: 10pt Tahoma; margin: 10px; }
+
+    .card {
+        border: 1px solid #767676;
+        padding: 10px;
+        margin-bottom: 12px;
+    }
+
+    h3 { margin: 0 0 6px 0; font-size: 12pt; }
+    .note { color: #555555; font-size: 8pt; }
+
+    /* --- prefers-color-scheme -------------------------------------------------
+       The light styling below is the baseline. On GDI+ and WPF the adapter reports
+       the Windows app theme, so switching Windows to Dark and reloading should
+       flip this card to the dark palette. PdfSharp always reports light. */
+    .scheme {
+        background-color: #ffffff;
+        color: #111111;
+        border-color: #cccccc;
+    }
+    .scheme .verdict:after { content: "light"; }
+
+    @media (prefers-color-scheme: dark) {
+        .scheme {
+            background-color: #1e1e1e;
+            color: #f0f0f0;
+            border-color: #555555;
+        }
+        .scheme .verdict:after { content: "dark"; }
+    }
+
+    /* --- media type ----------------------------------------------------------
+       The adapter also supplies the media type: "screen" for GDI+/WPF, "print"
+       for PdfSharp. Exactly one of these two cards should be visible. */
+    .screen-only { display: none; }
+    .print-only { display: none; }
+
+    @media screen {
+        .screen-only { display: block; background-color: #e8f4ff; }
+    }
+
+    @media print {
+        .print-only { display: block; background-color: #f4f4f4; }
+    }
+
+    /* --- feature evaluation --------------------------------------------------
+       A feature the matcher cannot evaluate drops its whole block, so this card
+       must never turn red. */
+    @media all and (scan: interlace) {
+        .unsupported-feature { background-color: #ff0000; color: #ffffff; }
+    }
+
+    /* --- viewport width ------------------------------------------------------
+       Evaluated against the container's available width. */
+    .width-probe { background-color: #f0f0f0; }
+    @media (min-width: 400px) {
+        .width-probe { background-color: #e6ffe6; }
+        .width-probe .verdict:after { content: "viewport is at least 400px wide"; }
+    }
+    @media (max-width: 399px) {
+        .width-probe .verdict:after { content: "viewport is under 400px wide"; }
+    }
+</style>
+</head>
+<body>
+
+<div class="card scheme">
+    <h3>prefers-color-scheme</h3>
+    This card follows the platform colour scheme. Reported scheme: <b class="verdict"></b>.
+    <div class="note">GDI+ and WPF read the Windows app theme; PdfSharp always reports light.</div>
+</div>
+
+<div class="card screen-only">
+    <h3>@media screen</h3>
+    Visible because the adapter's media type is <b>screen</b>.
+</div>
+
+<div class="card print-only">
+    <h3>@media print</h3>
+    Visible because the adapter's media type is <b>print</b>.
+</div>
+
+<div class="card unsupported-feature">
+    <h3>Unsupported media feature</h3>
+    This card stays neutral. A media feature the matcher cannot evaluate drops its whole block
+    rather than applying the rules inside it, so the red styling in
+    <code>@media all and (scan: interlace)</code> must not win.
+</div>
+
+<div class="card width-probe">
+    <h3>Viewport width</h3>
+    <span class="verdict"></span>
+</div>
+
+</body>
+</html>

Source/HtmlRenderer.PdfSharp/Adapters/PdfSharpAdapter.cs

@@ -67,6 +67,22 @@ public static PdfSharpAdapter Instance
             get { return _instance; }
         }
 
+        /// <summary>
+        /// Paged output, so @media print applies and @media screen does not.
+        /// </summary>
+        public override string DefaultMediaType
+        {
+            get { return "print"; }
+        }
+
+        /// <summary>
+        /// A PDF has no system theme to follow, so prefers-color-scheme always reports light.
+        /// </summary>
+        public override RColorScheme SystemColorScheme
+        {
+            get { return RColorScheme.Light; }
+        }
+
         /// <summary>
         /// Get the FontResolver instance for advanced font management.
         /// </summary>

Source/HtmlRenderer.WPF/Adapters/WpfAdapter.cs

@@ -71,6 +71,12 @@ private WpfAdapter()
 	            {
 	            }
             }
+
+            SystemEvents.UserPreferenceChanged += (sender, e) =>
+            {
+                if (e.Category == UserPreferenceCategory.General)
+                    _colorScheme = null;
+            };
         }
 
         /// <summary>
@@ -81,6 +87,25 @@ public static WpfAdapter Instance
             get { return _instance; }
         }
 
+        /// <summary>
+        /// Rendering onto a Windows surface, so the document should follow the user's app theme.
+        /// Cached and invalidated on a system preference change rather than read per query.
+        /// </summary>
+        public override RColorScheme SystemColorScheme
+        {
+            get
+            {
+                if (!_colorScheme.HasValue)
+                    _colorScheme = WindowsTheme.GetAppsColorScheme();
+                return _colorScheme.Value;
+            }
+        }
+
+        /// <summary>
+        /// Cached app theme; null when it needs to be re-read.
+        /// </summary>
+        private RColorScheme? _colorScheme;
+
         protected override RColor GetColorInt(string colorName)
         {
             // check if color name is valid to avoid ColorConverter throwing an exception

Source/HtmlRenderer.WPF/Utilities/WindowsTheme.cs

@@ -0,0 +1,54 @@
+// "Therefore those skilled at the unorthodox
+// are infinite as heaven and earth,
+// inexhaustible as the great rivers.
+// When they come to an end,
+// they begin again,
+// like the days and months;
+// they die and are reborn,
+// like the four seasons."
+//
+// - Sun Tsu,
+// "The Art of War"
+
+using Microsoft.Win32;
+using TheArtOfDev.HtmlRenderer.Adapters;
+
+namespace TheArtOfDev.HtmlRenderer.WPF.Utilities
+{
+    /// <summary>
+    /// Reads the Windows app theme, which is what <c>prefers-color-scheme</c> reports for on-screen
+    /// rendering.
+    /// </summary>
+    internal static class WindowsTheme
+    {
+        private const string PersonalizeKey = @"Software\Microsoft\Windows\CurrentVersion\Themes\Personalize";
+        private const string AppsUseLightTheme = "AppsUseLightTheme";
+
+        /// <summary>
+        /// The user's app theme, or <see cref="RColorScheme.Light"/> when it cannot be determined -
+        /// light is the Windows default and the safer assumption for a document that declares no dark
+        /// styling of its own.
+        /// </summary>
+        public static RColorScheme GetAppsColorScheme()
+        {
+            try
+            {
+                using (var key = Registry.CurrentUser.OpenSubKey(PersonalizeKey))
+                {
+                    if (key != null)
+                    {
+                        var value = key.GetValue(AppsUseLightTheme);
+                        if (value is int)
+                            return (int)value == 0 ? RColorScheme.Dark : RColorScheme.Light;
+                    }
+                }
+            }
+            catch
+            {
+                // A locked-down or missing key just means "no preference expressed".
+            }
+
+            return RColorScheme.Light;
+        }
+    }
+}

Source/HtmlRenderer.WinForms/Adapters/WinFormsAdapter.cs

@@ -48,6 +48,12 @@ private WinFormsAdapter()
             {
                 AddFontFamily(new FontFamilyAdapter(family));
             }
+
+            Microsoft.Win32.SystemEvents.UserPreferenceChanged += (sender, e) =>
+            {
+                if (e.Category == Microsoft.Win32.UserPreferenceCategory.General)
+                    _colorScheme = null;
+            };
         }
 
         /// <summary>
@@ -58,6 +64,26 @@ public static WinFormsAdapter Instance
             get { return _instance; }
         }
 
+        /// <summary>
+        /// Rendering onto a Windows control, so the document should follow the user's app theme.
+        /// Cached and invalidated on a system preference change rather than read per query.
+        /// </summary>
+        public override RColorScheme SystemColorScheme
+        {
+            get
+            {
+                if (!_colorScheme.HasValue)
+                    _colorScheme = WindowsTheme.GetAppsColorScheme();
+                return _colorScheme.Value;
+            }
+        }
+
+        /// <summary>
+        /// Cached app theme; null when it needs to be re-read.
+        /// </summary>
+        private RColorScheme? _colorScheme;
+
+
         protected override RColor GetColorInt(string colorName)
         {
             var color = Color.FromName(colorName);

Source/HtmlRenderer.WinForms/Utilities/WindowsTheme.cs

@@ -0,0 +1,54 @@
+// "Therefore those skilled at the unorthodox
+// are infinite as heaven and earth,
+// inexhaustible as the great rivers.
+// When they come to an end,
+// they begin again,
+// like the days and months;
+// they die and are reborn,
+// like the four seasons."
+//
+// - Sun Tsu,
+// "The Art of War"
+
+using Microsoft.Win32;
+using TheArtOfDev.HtmlRenderer.Adapters;
+
+namespace TheArtOfDev.HtmlRenderer.WinForms.Utilities
+{
+    /// <summary>
+    /// Reads the Windows app theme, which is what <c>prefers-color-scheme</c> reports for on-screen
+    /// rendering.
+    /// </summary>
+    internal static class WindowsTheme
+    {
+        private const string PersonalizeKey = @"Software\Microsoft\Windows\CurrentVersion\Themes\Personalize";
+        private const string AppsUseLightTheme = "AppsUseLightTheme";
+
+        /// <summary>
+        /// The user's app theme, or <see cref="RColorScheme.Light"/> when it cannot be determined -
+        /// light is the Windows default and the safer assumption for a document that declares no dark
+        /// styling of its own.
+        /// </summary>
+        public static RColorScheme GetAppsColorScheme()
+        {
+            try
+            {
+                using (var key = Registry.CurrentUser.OpenSubKey(PersonalizeKey))
+                {
+                    if (key != null)
+                    {
+                        var value = key.GetValue(AppsUseLightTheme);
+                        if (value is int)
+                            return (int)value == 0 ? RColorScheme.Dark : RColorScheme.Light;
+                    }
+                }
+            }
+            catch
+            {
+                // A locked-down or missing key just means "no preference expressed".
+            }
+
+            return RColorScheme.Light;
+        }
+    }
+}

Source/HtmlRenderer/Adapters/RAdapter.cs

@@ -80,6 +80,25 @@ protected RAdapter()
             _fontsHandler = new FontsHandler(this);
         }
 
+        /// <summary>
+        /// The CSS media type this adapter renders for, used to evaluate <c>@media</c> queries -
+        /// <c>"screen"</c> for on-screen adapters (the default), <c>"print"</c> for paged output.
+        /// </summary>
+        public virtual string DefaultMediaType
+        {
+            get { return "screen"; }
+        }
+
+        /// <summary>
+        /// The colour scheme the rendering surface presents, answering the <c>prefers-color-scheme</c>
+        /// media feature. Defaults to <see cref="RColorScheme.Light"/>; an adapter that renders onto a
+        /// themed surface should report the system setting instead.
+        /// </summary>
+        public virtual RColorScheme SystemColorScheme
+        {
+            get { return RColorScheme.Light; }
+        }
+
         /// <summary>
         /// Get the default CSS stylesheet data.
         /// </summary>

Source/HtmlRenderer/Adapters/RColorScheme.cs

@@ -0,0 +1,32 @@
+// "Therefore those skilled at the unorthodox
+// are infinite as heaven and earth,
+// inexhaustible as the great rivers.
+// When they come to an end,
+// they begin again,
+// like the days and months;
+// they die and are reborn,
+// like the four seasons."
+//
+// - Sun Tsu,
+// "The Art of War"
+
+namespace TheArtOfDev.HtmlRenderer.Adapters
+{
+    /// <summary>
+    /// The colour scheme the rendering surface presents, as reported by the platform adapter and
+    /// queried by the <c>prefers-color-scheme</c> media feature (Media Queries 5 §5.1).
+    /// </summary>
+    public enum RColorScheme
+    {
+        /// <summary>
+        /// A light surface - dark content on a light background. The default for any adapter that has
+        /// no system theme to follow.
+        /// </summary>
+        Light,
+
+        /// <summary>
+        /// A dark surface - light content on a dark background.
+        /// </summary>
+        Dark
+    }
+}