While you can use CSS with Flex, border doesn't seem to have any effect on S|Button at least for me. See Using Cascading Style Sheets.
example:
S|Button
{
background-image: Embed(source="assets/img-tree.png");
background-size: "100%";
border: 5px solid red:
}
So you need to look into Spark skinning: There's a simple example for a button skin on the Adobe devnet. For your needs you'll need to modify the Rect color and fill plus add a s:BitmapImage for the background. See: Introducing skinning in Flex 4 at Adobe devnet
Notice that the syntax is familiar to regular Flex syntax.