Add support for custom CSS

Sources/Adwaita/View/Modifiers/AppearObserver.swift

@@ -64,4 +64,23 @@ extension View {
         }
     }
 
+    /// Add CSS classes to the app as soon as the view appears.
+    /// - Parameter getString: Get the CSS.
+    /// - Returns: A view.
+    public func css(_ getString: @escaping () -> String) -> View {
+        inspectOnAppear { _ in
+            let provider = gtk_css_provider_new()
+            gtk_css_provider_load_from_string(
+                provider,
+                getString()
+            )
+            let display = gdk_display_get_default()
+            gtk_style_context_add_provider_for_display(
+                display,
+                provider?.opaque(),
+                .init(GTK_STYLE_PROVIDER_PRIORITY_APPLICATION)
+            )
+        }
+    }
+
 }

Tests/DiceDemo.swift

@@ -28,6 +28,14 @@ struct DiceDemo: View {
             }
             .pill()
             .suggested()
+            .style("dice-button")
+            .css {
+                """
+                .dice-button {
+                    background-color: @green_5;
+                }
+                """
+            }
             .frame(maxWidth: 100)
         }
         .valign(.center)