msglist [nfc]: Comment with TODO(#95) on mark-as-read button

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -475,6 +475,7 @@ class MarkAsReadWidget extends StatelessWidget {
             padding: const EdgeInsets.symmetric(horizontal: 10, vertical: 10 - ((48 - 38) / 2)),
             child: FilledButton.icon(
               style: FilledButton.styleFrom(
+                // TODO(#95) need dark-theme colors (foreground and background)
                 backgroundColor: _UnreadMarker.color,
                 minimumSize: const Size.fromHeight(38),
                 textStyle: