Is it possible to make a fixed Wide Box on top in Sparkle

I would like to have a fixed Wide Box at the top of the site, so that the Letterbox and the text under slide underneath when scrolling down. The logo and menu will reside in the Wide Box on top, so this way these would be all of the time.

In short: exactly like on this forum, where the Sparkle logo, the search option, the menu-button etc. all also reside in such a Wide Box.

Is this possible in Sparkle?