Flag to disable dark mode

#632 introduced dark mode. While I personally use dark mode at the OS level, I do not want dark mode on a ECMA standard. IMO, it does not look professional or authoritative.

I would like a way to disable dark mode, perhaps with a flag passed to ecmarkup. The only way I have found to revert the dark mode is to redefine the CSS variables.

This excerpt simply forces the original (light) color scheme for dark mode. But really, users should not have to do this.

@supports (color-scheme: dark) {
    @media (prefers-color-scheme: dark) {
      :root {
        --foreground-color: #111;
        --background-color: #fff;

        --link-foreground-color: #206ca7;
        --link-hover-foreground-color: #239dee;

        --user-code-foreground-color: brown;

        --var-foreground-color: #218379;

        --referenced0-background-color: #ffff6c;
        --referenced1-background-color: #ffa4a8;
        --referenced2-background-color: #96e885;
        --referenced3-background-color: #3eeed2;
        --referenced4-background-color: #eacfb6;
        --referenced5-background-color: #82ddff;
        --referenced6-background-color: #ffbcf2;

        --note-background-color: #e9fbe9;
        --note-border-color: #52e052;
        --note-editor-border-color: #faa;

        --nt-link-foreground-color: #333;

        --production-rhs-background-color: #f0f0f0;
        --production-rhs-border-color: #888;

        --params-foreground-color: #2aa198;
        --opt-foreground-color: #b58900;

        --title-foreground-color: #f60;

        --caption-foreground-color: #555555;

        --table-header-background-color: #eeeeee;

        --highlight-background-color-start: rgba(249, 241, 172, 1);
        --highlight-background-color-end: rgba(249, 241, 172, 0);

        --highlight-background-color: rgba(249, 241, 172, 1);

        --ins-background-color: #e0f8e0;
        --ins-border-color: #396;

        --del-background-color: #fee;

        --control-foreground-color: #eee;
        --control-dimmed-foreground-color: #666;
        --control-background-color: #ddd;
        --control-dark-background-color: #ccc;
        --control-border-color: #aaa;
        --control-light-border-color: #bbb;
        --control-shortcut-background-color: #eee;
        --control-shortcut-shadow-color: #ccc;
        --control-dialog-fade-color: rgba(255, 255, 255, 0.6);

        --control-link-foreground-color: #1567a2;

        --control-input-background-color: #bbb;
        --control-input-border-color: #999;

        --menu-header-background-color: #bbb;
        --menu-revealed-link-foreground-color: #206ca7;
        --menu-unpin-hover-foreground-color: #bb1212;

        --menu-trace-list-foreground-color: #999;
        --menu-trace-list-background-color: #222;

        --toolbox-tail-background-outside-color: rgba(0, 0, 0, 0);
        --toolbox-tail-border-outside-color: rgba(204, 204, 204, 0);

        --normative-optional-background-color: #ffeedd;
        --normative-optional-border-color: #ff6600;

        --attributes-tag-foreground-color: #884400;

        --figure-background: #fff;
      }
    }
  }

Start by tracing how ecmarkup emits dark-mode CSS and accepts options passed to the tool. Define the flag's expected behavior for generated documents, then verify that enabling it preserves the light color scheme without requiring users to override CSS variables.