I figured out that the .sdt file is a zip archive.
So I've opened some SXG75 theme files and they contain a "xxxx-skin.bar" file inside which contains theme's button images in png format.
Does anyone know how to make this .bar extension archive ?

Note: Actually I have a Motorola ve538 and it takes Siemens themes.Search the web to find why.

See More: How can I make my own themes for the SXG75