Theme per forum

I am trying to setup a theme for each forum.

When I define the theme per forum under the forums tab I do get the forums themed correctly but not the nodes under the forums.

I can use vocab-based template selection under the taxonomy tab to determine that all nodes of all forums are allocated to one particular theme.

When I look at term-based or all-taxonomy template selection under the taxonomy tab the terms for the forum vocabulary are not listed, presumably filtered out by the code.

I cannot find a way to allocate all nodes of a particular forum to the theme for that forum, so that a node belonging to a particular forum will be themed for that particular forum.

What am I missing?