All themes are located here:
http://pfsense.com/cgi-bin/cvsweb.cgi/pfSense/usr/local/www/themes/
Each directoryname is displayed as selectable theme in the dropdown at system>advanced (unless it starts with "_").
The all.css has all the colorinformations and the menu layout.
The other folders in the themedirectory hold the graphics for buttons, bars and so on.
For getting files up and down your pfsense enable ssh and logon with http://www.winscp.net using user root and password <webguipassword>.
Good luck.</webguipassword>