Mediator Theme - header image in post covers up content

Hi guys,
I found a bug in Mediator Theme. When you add header image to an article it covers up the whole post content. It’s not only on my sever, but in original demo as well - i.e.
I’m not a developer but I can change some things in CSS/twig/etc. to customize theme for my purpose. But this time I’ve got problem with fixing it. So maybe someone would be so kind and help me with it? Maybe there should be change in CSS only or maybe twig for post should be rebuild to new structure? Any ideas?