Attempt at a Swift class that listens to changes to the system accent color. Doesn't seem to work at the moment.

as_flutter_hex_string.swift

extension NSColor {
  var asFlutterHexString: String {
    let red = Int(round(self.redComponent * 0xFF))
    let green = Int(round(self.greenComponent * 0xFF))
    let blue = Int(round(self.blueComponent * 0xFF))
    let hexString = NSString(format: "#%02X%02X%02X", red, green, blue)
    return hexString.replacingOccurrences(of: "#", with: "0xFF") as String
  }
}

SystemAccentColorProvider.swift

//
//  SystemAccentColorProvider.swift
//  macos_ui
//
//  Created by Reuben Turner on 7/9/22.
//

import FlutterMacOS
import AppKit

class SystemAccentColorProvider: NSObject, FlutterStreamHandler {
  var eventSink: FlutterEventSink?
  var accentColor: NSColor?
  
  func onListen(withArguments arguments: Any?, eventSink events: @escaping FlutterEventSink) -> FlutterError? {
    eventSink = events
    return nil
  }
  
  @available(macOS 10.13, *)
  @objc public func startStream() {
    NotificationCenter.default.addObserver(
      self,
      selector: #selector(accentColorChanged),
      name: NSNotification.Name("AppleColorPreferencesChangedNotification"),
      object: nil)
    
    let hexColor = accentColor!.usingColorSpace(NSColorSpace.deviceRGB) ?? accentColor
    eventSink?(hexColor?.asFlutterHexString)
  }
  
  func onCancel(withArguments arguments: Any?) -> FlutterError? {
    eventSink = nil
    return nil
  }
  
  @available(macOS 10.13, *)
  @objc private func accentColorChanged(notification: NSNotification) -> NSColor {
    print(notification.userInfo!)
    let MacAccentColor = (NSColor.value(forKey: "controlAccentColor") as? NSColor ?? NSColor(named: "AccentColor")!)
    accentColor = MacAccentColor
    return MacAccentColor
  }
}

If anyone has any thoughts on why this doesn't work, please comment below.