I don't see any package called
emerald-themes in the Ubuntu repositories (https://launchpad.net/ubuntu/+source/emerald-themes).
You probably are getting this package from a PPA. Unfortunately, without knowing which PPA you are getting this
emerald-themes from, I can't help you further.
I see on ubuntuforums that a certain Ian-weisser suggested something (http://ubuntuforums.org/showthread.php?t=2172293) :
Here is one way I would try to solve it:
1) Disable all PPAs except the one containing emerald and
2) Uninstall the packages with sudo apt-get autoremove emerald
If they won't uninstall, then remove them manually with sudo dpkg
--remove emerald and sudo dpkg --remove emerald-themes
3) Clean out the offending packages from the cache with sudo apt-get
clean emerald emerald-themes
4) Reload and reinstall the packages with sudo apt-get install emerald
If they still won't install due to a dependency issue, then contact
the PPA maintainer directly. They need to fix at least one package.
I also suggest that you try thoses packages from Webupd8team : http://www.webupd8.org/2013/05/how-to-install-emerald-in-ubuntu-1304.html and have more luck.