Contempo theme - How can I get rid of the small white rectangles in post' cover pictures?

Question

we have the following blog:

https://atlantidivaganti.blogspot.it/

and we would like to know how to get rid of the white rectangles in the lower left corner of the post cover pictures that appear in the main page of the blog.

The only solution we've found was to set the background color as "transparent" following: Theme -> Customize/Personalize -> Advanced -> Background Color ("Colore sfondo" in Italian). But if we do so, the "Follow by Email" ("Iscriviti" in Italian) gadget appears transparent as well, and it is not easy to see.

I have prepared some slides to explain better the issue:

Thanks again in advance for your support and ciao,

Answer 1

Add the following CSS via Theme Customise Advanced Add CSS

.snippet-thumbnail .post-header {
    background: transparent;
}