You can install a new option for the window border setting in the Theme preference tool. Window border options reside in the /usr/share/themes/option-name/metacity-1 directory. The location of a window border option in the file system is as follows:
The option file for a window border option is located at /usr/share/themes/option-name/metacity-1/metacity-theme-1.xml.
The image files for a controls option in the file system are located at /usr/share/themes/option-name/metacity-1/*.*.
Typically, a new option for the window border setting is supplied as a .tar.gz file. To install the new window border option, unzip the .tar.gz file, then, untar the .tar file into the /usr/share/themes directory.
You can install your own options for the window border setting. If you install an option for the window border setting, the option is stored in the $HOME/.themes directory.