It seems that you have made changes in the theme core file, so please re-install the theme because if you upgrade your theme to pro version or free theme, then you will loose your all data. And regarding the banner which you have kept via sidebar-banner.php file, you can simply keep it with the help of banner widget, so please fix this and if you fix this then the gap between content and banner will be remove.
You can remove the gap with the help of css but you need to fix above things, we hope you understand. And here is the css to fix this issue:
NOTE: Please use the child theme style.css or custom css plugin to write the css.