You can take a look at different themes and see if they do what you want.
Otherwise, the easiest way is to style the element. Just inspect the green rectangle and see that it has a "formulation" class. So the css is simply:
.formulation {
background-color:transparent;
}
If you want this to apply to the entire site, assuming you are using the Boost theme, add the above code in Site administration / Appearance / Boost / Advance settings / Raw initial SCSS.