LiteStep is only BARELY multi-monitor capable. About the only thing that you can do reliably in a theme is to use positive coordinates for everything and stick to the primary display. (don't dock anything to the desktop / always on bottom) If you're a scripting genius, you might be able to reliably put stuff on other monitors, but you still have to ask the user where they are.

For the most part, it's not the themers' fault, it's the module coders' fault. Many problems do come down to theme bugs, but it should not be as hard as it is. Using negative coords is a theme bug? I don't think so.

Personally, I use multiple monitors (right now 3, but one is on another machine) so I can see my code (at least 2 files at once), project documentation, and API documentation all at the same time. Putting it all on one screen is a great way to make a 19" monitor feel like it's a 12".