Yes, but it's a PITA.

My method:
1) On a PC, go to here.
2) Go into the two carman-theme-?? folders and download the .deb files to your PC.
3) Rename both .deb files to .tar.gz files.
4) Use 7-zip to open the files and keep going through the folder tree until you get to the ones in the themes folder - drag these onto your desktop (or wherever).
5) You should now have two folders, one called eclipse and one called offroad.

6) At this point methods vary, but you need to get the folders into \usr\share\carman\themes on your n900.

7) I copied the folders via usb onto MyDocs, then "sudo gainroot"ed, created the requisite folders in the \usr\share\carman\themes folder, and copied the contents of the two folders into there.