~$ sudo apt-get install emerald-themes
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package emerald-themes is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package emerald-themes has no installation candidate
sudo apt-get update - completed with no errors
~$ sudo apt-get install emerald-themes
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package emerald-themes is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package emerald-themes has no installation candidate