Class variables act as read-only storage for class-constants. They 
are normally used for storing setting information, such as fonts, 
colours etc. For this reason, the default value for a class_variable 
is defined with the declaration of it, but this default my be overruled 
using the Defaults file. The system defaults file is 
located in the XPCE home directory (`@pce<-home'). 
This file contains an
include statement, including the file  /.xpce/Defaults,10On 
Windows systems,   expands to %HOME%, %USERPROFILE%, %HOMEDRIVE%/%HOMEPATH% 
or the root of the current drive. See expand_file_name/2 
of the SWI-Prolog manual. which may be used by the 
developer and application user to specify defaults.
Many XPCE built-in classes define class-variables. These can be examined using the ClassBrowser (see section 3.3.1) from the online manual tools.