Black rectangle underneath the text

Hello guys,

The question is pretty simple: I have the Full Frame theme installed on my WordPress, at https://www.dynamitri.pl. I have an issue with the text, though – as you can see, it is partially illegible if there are white/bright elements to the photo in the background. How could I go about adding a black rectangle underneath/behind the text (both header and the entry preview), so that it’s fully legible?

Also, I want the rectangle to show up on the homepage only. After clickthrough, I would like this to display as it is now. Any tips? Every kind of guidance would be greatly appreciated.

Many thanks.

Kind regards,

Dynamitri