Firstly, it would be a good idea to update to the newest version of litestep.

If you have not done so already the most recent version can be found @ http://beyondconvention.net/ohussain/lsinstaller/

Once you have the newest version, installing and changing themes is very simple. So long as the themes are OTS2 it should be incredibly easy to install/change themes. Especially with the advent of Omar's theme installer. Which, under the new version of litestep at the link above can be found in your /litestep/utilities/ and aptly named as lsthemeinstaller.exe

Good Luck,