The biggest difference between the two is Microsoft Longhorn themes are mostly binary, whereas Windows XP themes are largely plaintext. A theme can have multiple sizes and color schemes defined, and the theme engine will load your desired size/color scheme.
The underlying principles of the theme format, however remained the same. The theme format (.msstyles) changed fundamentally compared to Windows XP, even with the earliest leaked build of Microsoft Longhorn. INIs and resources (like Windows XP themes).
msstyles decompiler, that can turn compiled packthem v4 themes into. I did a bit of research on Longhorn's theme format, and figured out some things perhaps never fully documented before.
