PocketPC Themes let you personalize appearance of pages Today and Start. Their file extension is ".TSK". These files are in fact CAB files for installation on Pocket PC, renamed to TSK. They respect the standards of files CAB defined by Microsoft. A file CAB (and thus TSK) includes in fact one or more files.
ThemeGenCE helps you to build the TSK by compiling the information file (Extension "000") and by building the images which will be attached to your theme. It puts the whole in a CAB file renamed in TSK.