How to add a new theme?

Ok, read all posts but still no one has said how to add a new theme. I want to modify the one I have but not take a chance of ruining it. How do I go about duplicating the current theme and adding it as a second one that I can modify? I'm used to postnuke where all themes are in their own folders so changes to a theme do not effect others. Is vb like this? What files are involved in the theme? I have modified many many of the templates. Do these modifications carry over to all future themes? or do I have to modify these again as well. Step by step instructions would be great, but any help is appreciated.

I am trying to build a theme that would fit our extreme custom postnuke site. www.annointed.net and vbulletin <not live yet> is at http://connect.samstech.net We are almost done building a custom bridge for the user db without crones, the only thing holding us back is integrating the look.
thanks in advance

[Razor]

By theme, I assume you mean style? If so, to add a new one, go into the admin cp. Go down to where it says "styles", and select "add style". If you want to have it the same style as your current one, then click "download/upload", and make it through there.

[dreamhost]

thanks, i was able to click new style default default, then i went in and modified "testing2" style, but it modified default template as well. Can I ask how I copy my current style as a backup so i can play with it behind the scenes keeping all the hacks etc? not sure what settings to choose upon making new style
thanks

[Razor]

To copy your current style as a backup, go to the download options, and download the .style for the set. Choose the style you want to back up from the dropdown.