I found these icons for Dropbox that match the Ubuntu mono dark (ambiance) theme. The link includes instructions for installing the icons for Dropbox 0.8, but Dropbox is up to 1.0 now and the path and filenames have changed. The new path is:
and the filenames have all been prefixed with “dropboxstatus-“. With these minor tweaks, my Dropbox indicator now matches nicely with the rest of the Ambiance theme.