Making the GNOME top panel transparent is something a lot of users like to do — but it can look a little out of place when you maximise a window.
Dynamic TopBar is a nifty little extension for the GNOME Shell desktop that “makes the top bar transparent when no window is maximized.”
And when one is it turns to black (or whatever colour your GNOME Shell theme specifies, assuming it styles the background-color property of the panel and doesn’t use an image file).
Through the extension’s preferences you can customise the exact degree of transparency and choose whether or to there’s a button shadow.
Revolutionary? No, but it’s a neat behaviour. The app shelf on Chrome OS does a similar thing: transparent when no window is maximised, solid black when one is. There is probably a valid design rationale written up somewhere online for why this is good UX but i just think it looks nicer!
Dynamic TopBar is a free extension and works with GNOME 3.18 and up.