No dark theme possible in Ubuntu 16.04?

Hiya,

I'm using Ubuntu 16.04.2, and unfortunately Calibre 3.1.1 is not giving me any options to change to Ubuntu's default Ambiance dark theme. The only options I get at the User Interface Layout options are 'Narrow' and 'Wide'.

Is there an easy way (as in: not having to compile Calibre myself) to make Calibre change 'to the dark side' so to speak?

Thanks in advance for an already amazing e-book application. :)

PS: the new Calibre icon in Unity's application bar is not only barely appealing, it's also barely visible. It is too transparent in my view. Calibre deserves a better icon than that.