Well, I don't know why the .debs won't work for certain themes (like NuvoBlack), but if you extract the files from them, and copy them to the appropriate locations manually, it works just fine.

If anyone's curious, here's how I did it (I'll use NuvoBlack.deb as an example):

Create a new directory, and put the theme (in .deb form) in it:
Move into that directory:
Extract the files:
Copy the files where they need to go (note the periods in this command, they're important:
You're done! Activate the theme like you normally would.

Feel free to let me know if I've overlooked anything.