Add a transparent background on a specific, individual contact form

Hi there,
I posted a question on another topic started but didn’t receive a response and the topic was closed : https://kriesi.at/support/topic/change-contact-form-settings-individually-on-different-pages/

Here’s my original question:
I’m trying to add a background to a Contact Form on my home page banner. I’ve got a 2/3 and a 1/3 element inside a Color Section with a large background image (like on the Enfold theme demos) and have place a Form in the 1/3 element. I want that form to have a white but slightly transparent background. I’m not a savvy coder but I’ve gotten as far as dragging the code blog to the same 1/3 element as the contact form, and I’m hoping you can help me put together the basic code snippet to add that transparent box behind the form.

I was then asked for some screenshots:
I’m in dev mode right now but I’m trying to make it look like this: http://demo.whoathemes.com/crossway/v6/
Here’s a screnshot of how I’ve got the page set up: https://www.dropbox.com/s/26qh09uq019iry1/screenshot.jpg?dl=0

Any help you can provide would be appreciated!