`NSAppearance` change notification when you cannot use `NSApp.effectiveAppearance` which is available for macOS 10.14+ only

AppDelegate.swift

import Cocoa
import RxSwift
import RxCocoa

@NSApplicationMain
class AppDelegate: NSObject, NSApplicationDelegate {

    @IBOutlet weak var window: NSWindow!

    let disposeBag = DisposeBag()

    func applicationDidFinishLaunching(_ aNotification: Notification) {
        setUpAppearanceChangeNotifications()
        consumeAppearanceChangeNotificiations()
    }

    fileprivate func setUpAppearanceChangeNotifications() {
        window!.contentView!.rx
            .isDarkMode
            .startWith(NSAppearance.current.isDarkMode)
            .subscribe { isDarkMode in
                NotificationCenter.default.post(
                    name: NSView.darkModeDidChange,
                    object: nil,
                    userInfo: [NSAppearance.isDarkModeKey : isDarkMode])
            }.disposed(by: disposeBag)
    }

    fileprivate func consumeAppearanceChangeNotificiations() {
        let darkModeChange = NotificationCenter.default.rx
            .notification(NSView.darkModeDidChange)
            .map { $0.userInfo?[NSAppearance.isDarkModeKey] as? Bool }
            .filter { $0 != nil }.map { $0! }

        darkModeChange.debug("Dark Mode Change Signal").subscribe().disposed(by: disposeBag)
    }
}

DarkModeNotification.swift

import Cocoa

extension NSView {
    static var darkModeDidChange: Notification.Name { return .init(rawValue: "NSAppearance did change") }
}

extension NSAppearance {
    static var isDarkModeKey: String { return "isDarkMode" }
}

NSAppearance+DarkMode.swift

import Cocoa

// Adapted from https://indiestack.com/2018/10/supporting-dark-mode-checking-appearances/
extension NSAppearance {
    public var isDarkMode: Bool {
        if #available(macOS 10.14, *) {
            if self.bestMatch(from: [.darkAqua, .aqua]) == .darkAqua {
                return true
            } else {
                return false
            }
        } else {
            return false
        }
    }
}

NSView+Rx.swift

import Cocoa
import RxSwift

extension Reactive where Base: NSView {
    var effectiveAppearance: ControlEvent<NSAppearance> {
        let source = base.rx
            .observe(NSAppearance.self, "effectiveAppearance", options: [.new])
            .map { $0 ?? NSAppearance.current }
        return ControlEvent(event: source)
    }

    var isDarkMode: ControlEvent<Bool> {
        return base.rx.effectiveAppearance
            .map { $0.isDarkMode }
    }
}