if i take that image (from the default skin) and change color in photoshop (just add color overlay) and then use it for box style then that image is streched across the box it does not look good at all...how to do this properly?
First make sure that your texture is imported with texture type GUI. If you create your own style you might have to adjust the border property. It defines how many pixels from each side are fix. The center area gets scaled.