Good question.
First a quick way for you to explain which forum page you are on. The 3 main ones are the Group View page which is what you see or may call the forum home page.
Next is Forum View which is what I believe the page you are talking about. Then Topic View which is this page. The names used are also similar as the php files that create those views in your theme. spGroupvVew.php, spForumView.php and spTopicView.php.
Looking at the breadcrumbs it would be like this:
In regards to your question and using the example of what you see here. The red background in Forum View is not an independent header and then the the elements added below it. It is actually a red background that goes all the way down under the full list of topics. Then the topic list and other stuff are layed on top of the red background.
So the top where you see the icon, the RSS button and the name/description is only as tall as the info it shows.
Using here as an example, if the forum description was removed the red area would shrink because it does not have to show that line.
If you are displaying a forum description you could try turning that off first. If not enough you could remove or reduce the size of the icon and then maybe reduce the size of the text for the forum description.