You need to set up the option
under setup_options.php. It will switch to the "modern" layout.
However miniBB comes in the flat-style layout by default not by occasion. The "modern" layout will work only when you have enough topics to provide such wide-view of forums. Until your forums are not filled up with any topics, but you would like to start them right away, it's better keep the old-school schema.
However it's up to you. Fill in the forum with at least 20 topics immediately, and you'll have it.