there is not a guide or Glossary for the explanations, but forabg is the forumlist , the "blue" round corner of the forums on the index page, the headerbar is the header the blue header with the sitename into it.
The tabels are defined on viewonline, memberlist.php page , the blue "round" corner table.
The best thing to search is on .html files with the css clases like forumlist , headerbar etc to know what it is doing.
Normaly if you whant to change colours you go to colours.css , if you whant to change the posting page , viewtopic text, profile etc you go to content.css , i hope that you get the idea of it
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