The default Flex application look is called Halo. Flex includes a default stylesheet (default.css) that defines the default look of your Flex application in the framework.swc file found under the FLEX_HOME \frameworks\libs folder. The default.css file defines the look and feel of all Flex components, and is explicitly bundled with your application when you compile it.
The default Flex application color theme is called haloBlue. A color theme defines the default color scheme used for Flex components, such as a Button's mouse over, mouse click text color, focus rectangle color, and so on. You can change the default theme color by using Application tag's themeColor property, for example themeColor="haloGreen", themeColor="haloOrange", or themeColor="haloSilver", or you can set it to the color of your own choice.
Flex supports the use of external CSS. You can define your own stylesheet file with the .css extension. The general style declaration syntax is as follows...