Here's how I detect Photoshop's skin color and set my assets/colors accordingly

ApplicationSkin.as

package classes {
  public class ApplicationSkin {
    import com.adobe.csxs.core.CSXSInterface;
    import com.adobe.csxs.types.AppSkinInfo;  
    
    public function ApplicationSkin() {}
    
    // The current lightness of the app's background on a scale of 0-1
    //
    // Returns a Number
    static public function appLightness():Number {      
      return CSXSInterface.instance.getHostEnvironment().data.appSkinInfo.panelBackgroundColor.color.rgb/16777215
    }
    
    // The current RGB value of the app's background.
    //
    // Returns a Number
    static public function appColor():Number {     
      return CSXSInterface.instance.getHostEnvironment().data.appSkinInfo.panelBackgroundColor.color.rgb
    }
  }
}

GuideGuideComponent.mxml

<?xml version="1.0" encoding="utf-8"?>
<mx:Canvas xmlns:mx="http://www.adobe.com/2006/mxml" creationComplete="init()">
  <mx:Script>
    <![CDATA[
      import classes.ApplicationSkin;
      import com.adobe.csxs.events.*;

      private function init():void {
        updateUI()
        CSXSInterface.getInstance().addEventListener(AppReskinEvent.APPLICATION_RESKIN, function(e:AppReskinEvent):void {
          updateUI()
        })
        CSXSInterface.getInstance().addEventListener(StateChangeEvent.WINDOW_RESIZE, function(e:StateChangeEvent):void {
          updateUI()
        })
      }
      
      private function updateUI():void {
        var appUILightness:Number = ApplicationSkin.appLightness()
        
        if(appUILightness>.5){
          // Light Mode
        } else {
          // Dark Mode
        }
      }
    ]]>
  </mx:Script>
</mx:Canvas>

You could take the appLightness test a step further and detect light-dark vs dark-dark etc based on the number value it returns. In my case I'm only testing whether it's light or dark.