Problem formatting sidebar in Twenty Fifteen theme

I have added an image into the sidebar with a transparent background so the main contant background shows through. For some reason the background is showing in red and whilst I can see the style doing this with the developers tools I cannot actually locate the style or override it with my stylesheet.

The location of my site is http://www.artyfartyco.co.uk and it will be fairly obvious once you see it.

Any help as to why I cant override this style with my stylesheet would be greatly appreciated.

Many thanks