I've found a two-step way to get the mer theme to work in the chroot.

• Edit /home/user/.gtkrc-2.0 and comment out any theme-related lines. Add the following line:
  
  
  Code:

  include "/usr/share/themes/liberty/gtk-2.0/gtkrc"

• Start the sapwood theme server (as user, not root) inside the chroot. I have no idea why it doesn't share the running sapwood server in Maemo; probably Nokia's version is proprietary somehow.
  
  
  Code:

  /usr/lib/sapwood/sapwood-server &

If you want to maintain a consistent theme throughout, then you need to install the same theme in OS2008 and the chroot. So far, this seems to be limited to the Titan theme, since the Liberty theme isn't available for OS2008 and Titan is the only third-party OS2008 themes in the Mer repository at the moment.