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There's more then one way to do what you want. First, the easy one:
When you look in the templates for the forum, or any other module for that matter, you'll see pretty much each table/td has a class. Those classes are defined in the theme css. Rename the class to something new, lets say 'Forumsomething' and add it to your css. Pick a colour for that class and except for the forum table/td everything else remains the same.
Another way is to let it load an entirly different theme for different modules:
My theme.html looks like this:
le="color: #000000"><?php <{if $xoops_pagetitle == Forum }> <{include file="themedir/themeforum.html"}> <{elseif $xoops_pagetitle == sections }> <{include file="themedir/themesections.html"}> <{else}> <{include file="themedir/thememain.html"}> <{/if}>
Now a seperate themefile will be loaded for each specified module. Then you can change what ever you want. Some newer modules however, add topic/news titles to the pagetitle, so it might take some hacking to remove that if you want to use this the easy way.