SYDNEY THEME – separate image for MOBILE header

I have been going crazy with this for nearly two weeks!
Basically, te Sydney theme is great and I’ve been able to work around hurdles here and there. However, the header is too small to read on a mobile – so is there any way I can upload a separate image as a header just for mobile device?
I have seen this is available for other themes and have looked at Custom CSS codes – and tried them! None of them work, presumably because there are slight differences in CSS codes for each theme.

Thank you in advance!