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If you were to include the category name (and maybe even the discussion ID) as classes on the <body> tag, that would give great power and flexibility for customising the layouts of specific categories and even individual discussions using CSS.
Something like this?
<body class="general-discussions discussion7856">
Then people could target anything on those pages using CSS. A basic example being:
It wouldn't affect those people who didn't want to use it, so there's no down-sides to doing this! :)
You could go further and include a class of the username of the person who started the discussion, so you can give custom design features to certain accounts... tho that may be going too far!