Unfortunately, this is something 7z is currently incapable of doing, so we'll need to use FUSE to mount the ISO temporarily.
Current plans of obtaining stuff from ISOs are:
Use 7z to list the ISO and gauge what version of Windows it is
If Windows XP, extract the necessary files out of I386 using 7z, followed by extracting those files out of the extracted files themselves
If Windows Vista or above, FUSE mount the ISO temporarily, and 7z the install.wim/esd directly to obtain files out of the OS
Collect the obtained files in a permanent directory, ready for reuse in later theme conversions
Extract resources as necessary for the conversion (the reason this isn't done immediately is to retrofit for possible future additions, like automatic icon set creation, and so on)
Unfortunately, this is something 7z is currently incapable of doing, so we'll need to use FUSE to mount the ISO temporarily.
Current plans of obtaining stuff from ISOs are: