Update: I totally misread your question, see below for better answer
Do this if you want to find the elements to change
It’s not directly related to your question, but the section on “Using Developer Tools for debugging”, can be translated to your use case, where you then would click on the various bits and pieces you’d like to change, and then copy out the CSS selectors for the parts you need.
Outline alignment in recent updating is bit of confusing - #12 by holroy
Then you should be able to change colors of the weekend, hopefully.
What did you add into your CSS snippet? I added this, and got it to change background color:
.calendar {
--color-background-weekend: green;
}