Unfortunately, the theming engine is entirely different between Sharepoint 2007 and 2010, so themes from one don't work in the other. If the site was upgrade from 2007 and still in v3 UI, it would be using the old theming engine (and thus the old themes), but new sites are created in v4 UI and thus only use the new THMX theme format. There isn't an automatic way to create a THMX file from a 2007 theme. You could potentially manually pick out the color scheme from the original theme and create a THMX from that to use with SP 2010.
http://pravahaminfo.blogspot.com/2011/05/customize-themes-in-sharepoint-2010.html