The LSCP theme switcher is not "incorporated in LS itself", it is an EXE of its own and is not included in the next version of LiteStep (0.24.7). In fact, it never was included in the "official" "LiteStep" at all, it's an Indie LS feature. Its biggest problem is that it deletes $litestepdir$step.rc and completely rewrites it if you switch themes, so anything you put in there is lost. It also lacks many of LSTS' features, such as the preview image. Now before you say you don't need the preview image read the next paragraph...

LS is about choices, and the LS Installer chose LSTS to switch themes. Other distros/standards may choose other theme switchers. That means if you create a theme and want it to work with the LS Installer you'll need to include a proper .thm file in your theme.

About creating the .thm files, as Egonz said, use LSTS.