MyBB Community Forums

Is it possible to do so the logo is another than the default in specified categories.

It should be for the category which is selected and for all the subforums, threads, announcements and such aswell.

Can this be done someone, else I have to make a new theme for each of my many categories.

You could do it by using MyBB's functionality allowing a specific theme to be forced for certain forums. You would have to duplicate your theme once for each forum where you want a different logo and change the logo.