msglist: Find web's dark-theme color for typing-indicator text; add to MessageListTheme

[chrisbobbe]

@PIG208, would you be able to do this, as a followup to your PR #790 which was merged? Here's the color that should be updated:

    return Padding(
      padding: const EdgeInsetsDirectional.only(start: 16, top: 2),
      child: Text(text,
        style: const TextStyle(
          color: HslColor(0, 0, 53), fontStyle: FontStyle.italic)));

[gnprice]

Cross-linking: this forms part of #95.

[chrisbobbe]

I'll go ahead and take this; it should be quick, and I'd like to get it done today if I can. 🙂