Fixed book centre alignment

For screens wider than 1562px the book becomes increasingly misaligned with the centre divider. I assume it's intended for the book to be always centred on the screen so I added a CSS class and some JS to fix this when the client's screen is large enough. 
Currently this sits in app.js in the controls() function - which might not be the most logical place to put the code with regards to what this function is for, so I'm open to suggestion if you want to put it elsewhere.