NOMERGE test dark-theme message content

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -587,7 +587,7 @@ class MessageItem extends StatelessWidget {
       child: _UnreadMarker(
         isRead: message.flags.contains(MessageFlag.read),
         child: ColoredBox(
-          color: Colors.white,
+          color: const HSLColor.fromAHSL(1, 0, 0, 0.15).toColor(),
           child: Column(children: [
             MessageWithPossibleSender(item: item),
             if (trailingWhitespace != null && item.isLastInBlock) SizedBox(height: trailingWhitespace!),

lib/widgets/theme.dart

@@ -9,7 +9,7 @@ ThemeData zulipThemeData(BuildContext context) {
   final designVariables = DesignVariables();
   return ThemeData(
     typography: zulipTypography(context),
-    extensions: [ContentTheme.light(context), designVariables],
+    extensions: [ContentTheme.dark(context), designVariables],
     appBarTheme: AppBarTheme(
       // Set these two fields to prevent a color change in [AppBar]s when
       // there is something scrolled under it. If an app bar hasn't been