Rock Star Pro Background image

Google Search Console has just identified 'New Mobile Usability issues detected for site https://www.mattfurness.co.uk/' I can only think it relates to the background image. It displays fine on my laptop but when viewed on my iphone I just get a plain black background which I believe is taken from the left half portion of the image. This suggests to me that this is what Search Console is detecting. Any thoughts? Is this a general issue with the Rock Star Pro theme? Many thanks, Chris