CSS - transparent background

Hello,

I'm trying to change the color of the background to 'transparent'. I'm using the "frame the webchat URL" option and I'd like chat messages to overlay over a background image.

I understand that I need to upload a custom css file; however, I can locate the correct style element. Any help is appreciated.

Also, if anyone in the community has worked with CSS files, I'd love to hire you to get some custom CSS help.