How to make transparent/semi opaque

I’m using the Graphene theme.

Is there a simple code that I could copy and paste into the custom CSS window to make all wrappers and backgrounds transparent or 50% transparent black so that I can see through to my custom background.

Here’s my site: https://66.147.244.240/~whalesin/temp/

I want it to look like this: https://whalesincubicles.com/

My knowledge of CSS is VERY limited so I’m looking for an over-explanation if possible.