I would say the "easy" which you DONT WANT.. is to copy your style download it under
another name, then reimport it with the changed logo.
then in adminCD > Forums & Moderators > Forum manager > Select specified forum >
under "Style options" Select your newly importet theme.
and then "Override Users' Style Choice " - Yes.
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A few tips,, when downloading and editing you just have to edit 1 folder
well 1 image. and then upload that ONE image with the rest of that folder
in style vars, you can set that folder usually "misc" to point at the new inportet
folder, while the rest still point at your old.
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tadaaa one forum have changed while none of the others have..
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Its not impossible doing it by code, i just dont have a clue