Flex中如何通过在全局selector中设置不同的样式

<?xml version="1.0" encoding="utf-8"?> 
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" 
        layout="vertical" 
        verticalAlign="middle" 
        backgroundColor="white"> 
  
    <mx:Style> 
        global { 
            modalTransparencyBlur: 0; 
            modalTransparency: 0.8; 
            modalTransparencyColor: black; 
            modalTransparencyDuration: 500; 
        } 
    </mx:Style> 
  
    <mx:Script> 
        <![CDATA[ 
            import mx.controls.Alert; 
            import mx.managers.PopUpManager; 
  
            private function showAlert():void { 
                Alert.show("hello", "world"); 
            } 
  
            private function showContactForm():void { 
                var contactForm:ContactForm = new ContactForm(); 
                PopUpManager.addPopUp(contactForm, this, true); 
            } 
        ]]> 
    </mx:Script> 
  
    <mx:ApplicationControlBar dock="true"> 
        <mx:Button label="Alert" click="showAlert();" /> 
        <mx:Button label="ContactForm" click="showContactForm();" /> 
    </mx:ApplicationControlBar> 
  
</mx:Application>

如下是弹出的页面代码

<?xml version="1.0" encoding="utf-8"?> 
<mx:TitleWindow xmlns:mx="http://www.adobe.com/2006/mxml" 
        width="320" 
        height="240" 
        showCloseButton="true" 
        close="titleWindow_close();" 
        creationComplete="titleWindow_creationComplete();"> 
  
    <mx:Script> 
        <![CDATA[ 
            import mx.controls.Alert; 
            import mx.managers.PopUpManager; 
  
            private function titleWindow_close():void { 
                PopUpManager.removePopUp(this); 
            } 
  
            private function titleWindow_creationComplete():void { 
                PopUpManager.centerPopUp(this); 
            } 
  
            private function sendButton_click():void { 
                Alert.show("Thanks for the feedback"); 
                titleWindow_close(); 
            } 
        ]]> 
    </mx:Script> 
  
    <mx:Form styleName="plain" width="100%" height="100%"> 
        <mx:FormHeading label="Contact Us" /> 
        <mx:FormItem label="Name:" width="100%"> 
            <mx:TextInput id="feedbackName" width="100%" /> 
        </mx:FormItem> 
        <mx:FormItem label="Email:" width="100%"> 
            <mx:TextInput id="feedbackEmail" width="100%" /> 
        </mx:FormItem> 
        <mx:FormItem label="Comments:" width="100%" height="100%"> 
            <mx:TextArea id="feedbackComments" width="100%" height="100%" /> 
        </mx:FormItem> 
    </mx:Form> 
  
    <mx:ControlBar horizontalAlign="right"> 
        <mx:Button id="sendButton" 
                label="Send" 
                click="sendButton_click();" /> 
    </mx:ControlBar> 
  
</mx:TitleWindow>