Change the font of Reader, Firefox 57

Question

This look like a duplicate of How to change font of Reader View firefox? but none of the answers there work since Firefox Quantum

All points to userContent.css but none worked.

1. use @-moz-document from reddit

    @-moz-document url-prefix("about:reader") {
      #container {
        font-family: "courier new" !important;
        color: blue !important;

    }

Result: text didn't turn blue and font didn't change.

2. using :root selector from a blog

:root[hasbrowserhandlers="true"] body.dark.serif,
:root[hasbrowserhandlers="true"] body.dark.serif #reader-domain {
        font-family: "courier new" !important;
        color: blue !important;
}

Result: text didn't turn blue and font didn't change.

This is assembled by myself:

body.serif .moz-reader-content {
        font-family: "Helvetica" !important;
        color: red !important;
}

It achieved the change of text color, but the font remained. The inspector shows my rule "Helvetica" !important; takes precedence over the default rule, but the 'calculated' CSS rule at work is font-family: serif, "Helvetica"; with serif inserted before Helvetica with the all mighty hand of God.

If you achieved changing the font of Reader mode (without changing what defines serif by user preference please answer below.