Transparent Body/ Content Area

I am working on a website (https://www.bullcitybotics.co/) and I would like to change the content background to be 90% transparent and also if possible – have a shadow box border similar to the menu (on the left of the screen) on this page : https://demo.vellumwp.com/home-pages/one-page-resort/

I have installed simple custom css and I have tried a few things but I cannot seem to change that darn white rectangle – and I would like this on every page of the site as the background is static.

Any help is appreciated!

L