Desktop themes reside in the /usr/share/themes directory. The typical structure of a desktop theme in the file system is as follows:
Typically, a new desktop theme is supplied as a .tar.gz file. To install the new desktop theme, unzip the .tar.gz file, then untar the .tar file into the /usr/share/themes directory.
Users can install their own desktop themes. If a user installs a desktop theme, the theme is stored in the $HOME/.themes directory.