How to create transparent Background for form entire formbo using Contact Form 7

I’m trying to create a black transparent background for my contact box. I’ve seen the other posts that explain how to create a background, and while the workd, I would like for the form content to be centered in the actual box.

http://www.MartinaSally.com/Contact

You can’t read the font very well, on this page or others.

When I add the background to it (as explained by the other post) The form contents are aligned to the left. I would like for them to be centered so I guess I’m asking.

1. How to center CF7 form contents to the center of the form box
2. How to make it transparent by 50%

I’m a literal newbie so any info help

Thanks