LibWeb/CSS: Implement the light-dark color function

Author: https://github.com/Gingeh
Commit: 8e56109515
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3146
Reviewed-by: https://github.com/AtkinsSJ ✅

Libraries/LibWeb/CSS/StyleValues/CSSLightDark.cpp

@@ -0,0 +1,37 @@
+/*
+ * Copyright (c) 2025, Ladybird contributors
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "CSSLightDark.h"
+#include <LibWeb/Layout/Node.h>
+
+namespace Web::CSS {
+
+Color CSSLightDark::to_color(Optional<Layout::NodeWithStyle const&> node) const
+{
+    if (node.has_value() && node.value().computed_values().color_scheme() == PreferredColorScheme::Dark)
+        return m_properties.dark->to_color(node);
+
+    return m_properties.light->to_color(node);
+}
+
+bool CSSLightDark::equals(CSSStyleValue const& other) const
+{
+    if (type() != other.type())
+        return false;
+    auto const& other_color = other.as_color();
+    if (color_type() != other_color.color_type())
+        return false;
+    auto const& other_light_dark = verify_cast<CSSLightDark>(other_color);
+    return m_properties == other_light_dark.m_properties;
+}
+
+String CSSLightDark::to_string(SerializationMode mode) const
+{
+    // FIXME: We don't have enough information to determine the computed value here.
+    return MUST(String::formatted("light-dark({}, {})", m_properties.light->to_string(mode), m_properties.dark->to_string(mode)));
+}
+
+}