Flex - Style with CSS

Flex supports the use of CSS syntax and styles to apply styles to its UI controls in the same way as CSS to HTML components.

Way #1: Using external style sheet file

You can refer to a style sheet available in the class path of the application. For example consider Style.css file in com/tutorialspoint/client folder where HelloWorld.mxml file also lies.

/* CSS file */
@namespace s "library://ns.adobe.com/flex/spark";
@namespace mx "library://ns.adobe.com/flex/mx";
...
.container {
   cornerRadius :10;
   horizontalCenter :0; 
   borderColor: #777777;
   verticalCenter:0;
   backgroundColor: #efefef;
}

Then css file can be referred by following code snippet

<fx:Style source="/com/tutorialspoint/client/Style.css"/>

Assign styles to UI component using styleName property

<s:BorderContainer width="500" height="500" id="mainContainer" 
      styleName="container"> 
...
</s:BorderContainer>    

Way #2: Using styles within UI container component

You can define styles within UI container component using <fx:Style> tag

Class Level Selector

<fx:Style>
@namespace s "library://ns.adobe.com/flex/spark";
@namespace mx "library://ns.adobe.com/flex/mx";

/* class level selector  */
.errorLabel {
   color: red;
}  
</fx:Style>

Assign styles to UI component using styleName property.

<s:Label id="errorMsg" text="This is an error message" styleName="errorLabel"/>

Id Level Selector

Style UI component using id selector.

<fx:Style>
/* id level selector  */
#msgLabel {
   color: gray;
}
</fx:Style>

<s:Label id="msgLabel" text="This is a normal message" />

Type Level Selector

Style one type of UI Component in one go.

<fx:Style>
/* style applied on all buttons  */
s|Button { 
   fontSize: 15;
   color: #9933FF;
}
</fx:Style>

<s:Button label="Click Me!" id="btnClickMe" 
click="btnClickMe_clickHandler(event)"  />

Flex Style with CSS Example

Let us follow the following steps to check css styling of a Flex application by creating a test application:

Step	Description
1	Create a project with a name HelloWorld under a package com.tutorialspoint.client as explained in the Flex - Create Application chapter.
2	Modify Style.css, HelloWorld.mxml as explained below. Keep rest of the files unchanged.
3	Compile and run the application to make sure business logic is working as per the requirements.

Following is the content of the modified css file src/com.tutorialspoint/Style.css.

/* CSS file */
@namespace s "library://ns.adobe.com/flex/spark";
@namespace mx "library://ns.adobe.com/flex/mx";

.heading
{
   fontFamily: Arial, Helvetica, sans-serif;
   fontSize: 17px;
   color: #9b1204;
   textDecoration:none;
   fontWeight:normal;
}

.button {
   fontWeight: bold;   
}

.container {
   cornerRadius :10;
   horizontalCenter :0; 
   borderColor: #777777;
   verticalCenter:0;
   backgroundColor: #efefef;
}

Following is the content of the modified mxml file src/com.tutorialspoint/HelloWorld.mxml.

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009"
   xmlns:s="library://ns.adobe.com/flex/spark"
   xmlns:mx="library://ns.adobe.com/flex/mx"
   width="100%" height="100%" minWidth="500" minHeight="500"
   initialize="application_initializeHandler(event)">
   <!--Add reference to style sheet -->
   <fx:Style source="/com/tutorialspoint/client/Style.css"/>

   <!--Using styles within mxml file -->
   <fx:Style>
      @namespace s "library://ns.adobe.com/flex/spark";
      @namespace mx "library://ns.adobe.com/flex/mx";

      /* class level selector  */
      .errorLabel {
         color: red;
      }  

      /* id level selector  */
      #msgLabel {
         color: gray;
      }   

      /* style applied on all buttons  */
      s|Button { 
         fontSize: 15;
         color: #9933FF;
      }
   </fx:Style>
   <fx:Script>
      <![CDATA[
         import mx.controls.Alert;
         import mx.events.FlexEvent;
         protected function btnClickMe_clickHandler(event:MouseEvent)
         :void {
            Alert.show("Hello World!");    
         }

         protected function application_initializeHandler(event:FlexEvent)
         :void {
            lblHeader.text = "CSS Demonstrating Application";    
         }
      ]]>
   </fx:Script>
   <s:BorderContainer width="560" height="500" id="mainContainer" 
      styleName="container">
      <s:VGroup width="100%" height="100%" gap="50" 
         horizontalAlign="center" verticalAlign="middle">
         <s:Label width="100%" id="lblHeader" fontSize="40"
            color="0x777777" styleName="heading"/>
         <s:Button label="Click Me!" id="btnClickMe" 
            click="btnClickMe_clickHandler(event)"  /> 
         <s:Label id="errorMsg" 
            text="This is an error message" styleName="errorLabel" />   
         <s:Label id="msgLabel" text="This is a normal message" />
      </s:VGroup> 
   </s:BorderContainer> 
</s:Application>

Once you are ready with all the changes done, let us compile and run the application in normal mode as we did in Flex - Create Application chapter. If everything is fine with your application, this will produce following result: [ Try it online ]