That's a "context sensitive toolbar" so it's content changes depending on what you are doing. Does it disappear and the work area moves up into it's place or does it just turn gray or does it go black?
It will turn gray/tan when you are using a tool like the node-editor that doesn't have any extra settings until a node is clicked. Once you click on some part of a design the node-editor has something to work with and the toolbar populates with usable tools.
I occasionally have the entire upper set of toolbars turn black when I have too many large tabs open. The only way to get it back is to close down a few files.