Responsive Header not working – How to fix it?

Hi guys,
I am running the suits theme on my blog https://www.oliviaontheriviera.com/
My header image used to be responsive to mobile devices, ipads etc when the site was hosted on wordpress.com but ever since I moved to www.ads-software.com it no longer is, plus there are white bits on the side.
Can somebody please explain how to fix the responsiveness and white bits issue and WHERE I should do it exactly? I have created a child them and a style.css file but its not clear for me where one needs to make changes to code once the child theme is created. I’m a beginner!!
Thank you so much.