msglist [nfc]: Remove a TODO for finding some dark-theme colors

For the background color, the unread-marker color seems like a very reasonable starting point, since that's what we're using in light theme. White seems reasonable for dark theme, just like in light theme. There shouldn't be an issue with contrast; I expect the background will be darker, if anything, in dark theme. We can always refine these styles later.

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -534,7 +534,6 @@ class _MarkAsReadWidgetState extends State<MarkAsReadWidget> {
  // Give the buttons a constant color regardless of whether their
  // state is disabled, pressed, etc. We handle those states
  // separately, via MarkAsReadAnimation.
- // TODO(#95) need dark-theme colors (foreground and background)
  foregroundColor: WidgetStateColor.resolveWith((_) => Colors.white),
  backgroundColor: WidgetStateColor.resolveWith((_) => _UnreadMarker.color),
  ),