Added "#player" ID to wrapper element of video & chat (the .flex-container-row div)
Added ".theatre" styling for this "#player" element which changes the layout to a theatre mode layout
NB: this should be very responsive, I've changed chat width to a fixed width with min- and max-width as percentages so it should always be a good width
Added JavaScript in ControlBar.js to toggle theatre mode (mostly adapted from fullscreen JS)
Added some CSS to render plus and minus icons as theatre button, from icomoons (same package that's used for the other icons in the control bar)