Easiest way to implement custom css to change background colors?

I’m having a hard time isolating the code to change background colors. Obviously, I don’t want to edit the style.css, but want to include some lines in the custom css field to change all background colors. Essentially, I want to make most backgrounds either white or transparent, since my page background has now white.
Shouldn’t this be fairly easy to implement? Do I need to also include code to set box and background like this:

//
box-shadow: none

background border: none
//