You use the <mx:Style> tag to define styles that apply to the current document and its children. You define styles in the <mx:Style> tag using CSS syntax and can define styles that apply to all instances of a control or to individual controls.
If you reference a file by using the source property, that file must reside on the server and not on the client machine. The compiler reads the source value and compiles the source into the application at compile-time; the source value is not read at runtime.
MXML Syntax
The <mx:Style> tag has the following syntax:
<mx:Style [source="style_sheet"]>
[selector_name {
style_property: value;
[...]
}]
</mx:Style>