Please click here and read carefully through this blog post.
This code adds an extra section to your theme that you can use to replace the one you're using. It's free.
You are seeing this gray bar because, by default, Free themes' banner sections are not responsive. As you may have noticed unfortunately it's unlikely someone will help you any time soon with this issue in the community, there are only a few handful of developers active in here. So I reckon that this would be the best alternative to get your issue fixed ASAP.