Style documentation is very poor

**URL**
https://ionicframework.com/docs/api/modal#css-custom-properties

**What is missing or inaccurate about the content on this page?**
I mean I've been working with ionic since May in 2018 (Ionic 3 then, Ionic 4 since stable release came out) and honestly is very cumbersome to style elements. The documentation does not explain anything and what it explains it just doesn't work. most of the times I have to inspect the elements through the dev tools and see which one I'm supposed to style.

In the example I'm giving, I'm trying to add opacity to the background color of the modal but is very impossible to do. Documentation does not explain if this should be done to the page itself that the modal controller is hosting, or if there is some ion-modal element that we should tackle.