That works too!

    @-moz-document url-prefix(about:reader) {
      .container {
        column-count: 4;
        min-width: 85vw !important;
        margin: 0 50px 0 50px;
      }
    }

https://i.ibb.co/7XT4zfs/example-2.webp

It's chrome/userContent.css in the Firefox profile subdirectory, enabled by the about:config flag toolkit.legacyUserProfileCustomizations.stylesheets.