actually, I solved is by reverse thinking

In Joomla you can assign a default template that will be used for the webpages, you can assign another template to other pages, like the forum. The forum opened in the assigned template but when a topic was opened the default template is used again.

Nog I set the forum-template as default template, but assigned the template that I wanted for my default-template to all website-pages. A bit complicated, since I will now have to assign that template to whatever new page I create, but I do not often make new pages.

It seems to work fine, there are problems with the bridge though, but that's a different kind of topic.