Add support for specifying the CSS scheme to load

Sources/Adwaita/Model/Enumerations/ColorScheme.swift

@@ -0,0 +1,23 @@
+//
+//  ColorScheme.swift
+//  Adwaita
+//
+//  Created by david-swift on 01.02.26.
+//
+
+import CAdw
+
+/// The system color scheme.
+public enum ColorScheme: UInt32 {
+
+    /// The dark color scheme.
+    case dark = 2
+    /// The light color scheme.
+    case light = 3
+
+    /// The ColorScheme value as GtkInterfaceColorScheme.
+    var gtkValue: GtkInterfaceColorScheme {
+        .init(rawValue)
+    }
+
+}

Sources/Adwaita/View/AnyView++.swift

@@ -336,9 +336,11 @@ extension AnyView {
     }
 
     /// Add CSS classes to the app as soon as the view appears.
-    /// - Parameter getString: Get the CSS.
+    /// - Parameters:
+    ///     - scheme: The color scheme.
+    ///     - getString: Get the CSS.
     /// - Returns: A view.
-    public func css(_ getString: @escaping () -> String) -> AnyView {
+    public func css(scheme: ColorScheme? = nil, getString: @escaping () -> String) -> AnyView {
         inspect { storage, updateProperties in
             let cssID = "internal-css"
             let providerID = "internal-css-provider"
@@ -346,6 +348,9 @@ extension AnyView {
             let string = getString()
             if updateProperties, string != previous {
                 let provider = gtk_css_provider_new()
+                if let scheme {
+                    gtui_cssprovider_set_prefers_color_scheme(.init(Int(bitPattern: provider)), scheme.gtkValue)
+                }
                 gtk_css_provider_load_from_string(
                     provider,
                     string

Sources/CAdw/shim.h

@@ -85,3 +85,13 @@ gtui_initialize_boolean (gboolean boolean)
   g_value_set_boolean(&val, boolean);
   return val;
 }
+
+static void
+gtui_cssprovider_set_prefers_color_scheme (uint64_t provider, GtkInterfaceColorScheme scheme)
+{
+  GValue val = G_VALUE_INIT;
+  g_value_init(&val, G_TYPE_ENUM);
+  g_value_set_enum(&val, scheme);
+  g_object_set_property(provider, "prefers-color-scheme", &val);
+  g_value_unset(&val);
+}