LayerSlider – Dark Grey Background beside Images

We have placed a LayerSlider at the top of our website and we would like it to remain a fixed size of 300x1170px. However when we do that, we are left with a dark grey background bordering either side of the image. You can refer to the page here for reference: http://waterlooarchitecture.com/bridge/home-test-sidebar-2/

We have tried changing the CSS in the LayerSlider Skin Editor and in the CSS Editor. Is it possible to change this? If so how do we go about doing it?

Thank you.