Rework theme API

This commit adds support for theming on macOS and
also unifies the system theme handling across platforms.

src/platform_impl/macos/appkit/appearance.rs

@@ -0,0 +1,29 @@
+use objc2::foundation::{NSArray, NSObject, NSString};
+use objc2::rc::{Id, Shared};
+use objc2::{extern_class, extern_methods, msg_send_id, ClassType};
+
+extern_class!(
+    #[derive(Debug, PartialEq, Eq, Hash)]
+    pub(crate) struct NSAppearance;
+
+    unsafe impl ClassType for NSAppearance {
+        type Super = NSObject;
+    }
+);
+
+type NSAppearanceName = NSString;
+
+extern_methods!(
+    unsafe impl NSAppearance {
+        pub fn appearanceNamed(name: &NSAppearanceName) -> Id<Self, Shared> {
+            unsafe { msg_send_id![Self::class(), appearanceNamed: name] }
+        }
+
+        pub fn bestMatchFromAppearancesWithNames(
+            &self,
+            appearances: &NSArray<NSAppearanceName>,
+        ) -> Id<NSAppearanceName, Shared> {
+            unsafe { msg_send_id![self, bestMatchFromAppearancesWithNames: appearances,] }
+        }
+    }
+);