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,] }
+        }
+    }
+);

src/platform_impl/macos/appkit/application.rs

@@ -4,7 +4,7 @@ use objc2::runtime::Object;
 use objc2::{extern_class, extern_methods, msg_send_id, ClassType};
 use objc2::{Encode, Encoding};
 
-use super::{NSEvent, NSMenu, NSResponder, NSWindow};
+use super::{NSAppearance, NSEvent, NSMenu, NSResponder, NSWindow};
 
 extern_class!(
     #[derive(Debug, PartialEq, Eq, Hash)]
@@ -82,6 +82,13 @@ extern_methods!(
         #[sel(setMainMenu:)]
         pub fn setMainMenu(&self, menu: &NSMenu);
 
+        pub fn effectiveAppearance(&self) -> Id<NSAppearance, Shared> {
+            unsafe { msg_send_id![self, effectiveAppearance] }
+        }
+
+        #[sel(setAppearance:)]
+        pub fn setAppearance(&self, appearance: &NSAppearance);
+
         #[sel(run)]
         pub unsafe fn run(&self);
     }

src/platform_impl/macos/appkit/mod.rs

@@ -11,6 +11,7 @@
 #![allow(clippy::enum_variant_names)]
 #![allow(non_upper_case_globals)]
 
+mod appearance;
 mod application;
 mod button;
 mod color;
@@ -28,6 +29,7 @@ mod version;
 mod view;
 mod window;
 
+pub(crate) use self::appearance::NSAppearance;
 pub(crate) use self::application::{
     NSApp, NSApplication, NSApplicationActivationPolicy, NSApplicationPresentationOptions,
     NSRequestUserAttentionType,