Hello,

As for now, you can remove the black box picture in background by adding following given CSS in Additional CSS section by going to Admin Panel > Appearance > Customize > Additional CSS

#page-site-header{
  background-color: white;
}

#page-site-header .overlay{
  background-color: transparent;
}

#page-site-header .page-title{
 color: #000;
}

#breadcrumb-list span{
 color: #000;
}

.trail-items li:not(:last-child):after{
  color: #000;
}

Since the background color of the site is white, the blog post text is being invisible. So, we have added black color to the text. If you want to replace the black color with your desired color, replace the color code(#000) with your desired color code in above given CSS.

Hope this helps.

If you have any queries further, let us know.

Thank you.