Depends what you mean by desktop theme. What I suspect you mean is those such as the ones in the DesktopX section. The software you need for this is DesktopX. To get any of the software for which themes are available on this site, just click the application name in the top right of the boxes containing the screenshots/description in the theme sections. If you mean theming the windows, then there's 2 options, the more powerful of these is WindowBlinds, available the same way as the other applications on this site. The other way is with MSStyles, which are native to Windows XP. If you want to explore this route you'll first have to track down and install a hacked uxtheme.dll (searching for it on google should yield plenty of results).
Hope this helps,
AJ
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