The bright blue was just a placeholder – I’d likely make this thinner, grey, and with a nicer icon. 
And it shouldn’t be resizable – just a collapser. Something along these lines is probably a bit more subtle:
Not overly so, I think – it’s mainly changing the location of the “hide preview” link in the DOM, and some minor CSS styling.
From a user’s perspective, though, it is quite a large change, because rather than things just looking a little prettier, things will move – and so there could be some initial confusion.