Hello @ all,
i have edit the template like sebasvos says.
Then i have changed the template for the forum to the new one (with assign).
And that works. The Froum is displayd to the full width of the site.
But it's not every time full width.
When i make a new "post" and klick submit, then comes the page "Your message has been successfully posted" and after that the site reloads, and then the forum uses the old template. (and of cource not full width)
When i click again on some forum content(a forum entry, or on Forum, or on "show latest posts"

then it's again full width.
Pleace help!! How can i solve this.
Best regards