It is fairly easy with the Theme Creator Pro software. The Theme Creator Pro software is currently under Beta testing.

Theme Creator Pro is a powerful tool designed to create, edit, browse and manage themes for Sony Ericsson mobile phones. The user-friendly user interface and some smart functions permits you to work efficiently and create themes in minutes.
With the latest version of Theme Creator Pro you can create and preview themes for following phone models: T68/T68i/T68ieT226T230/T238T300/T302/T306T310/T312/T316T610/T616/T618T630/T628Z600/Z608

check it out at http://tcpro.lasyk.net/