AlertDialog background color issue

I have an Android AlertDialog XML layout that in the interest of better visibility sets the background to white and the textColor to black. This works fine until I remove the android:text="Hello World" entry and try to setMessage in Java to something meaningful. The meaningful message gets set and displayed fine but I lose the white background with black text... Can anybody help explain what's going on here (starting to pull my hair out!)? I have included the code below.

<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/scrollview_1" 
android:layout_width="fill_parent" 
android:layout_height="fill_parent"
android:background="@color/white">
<LinearLayout 
    android:id="@+id/layout_root"
    android:orientation="horizontal"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:padding="10dp">
    <TextView android:id="@+id/help_text" 
        android:layout_height="wrap_content"
        android:layout_width="wrap_content"
        android:text="Hello World" 
        android:textSize="20sp" 
        android:textColor="@color/black">
    </TextView>
</LinearLayout>

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private AlertDialog showHelp() {

    AlertDialog alertDialog = new AlertDialog.Builder(Game.this).create();
    View diagview = LayoutInflater.from(getBaseContext()).inflate(R.layout.help_dialog,    
                                            (ViewGroup) findViewById(R.id.layout_root));
    alertDialog.setTitle("Help...");
    alertDialog.setView(diagview);
    alertDialog.setIcon(R.drawable.ic_help);
    alertDialog.setMessage(this.getString(help[helpIndex]));
    alertDialog.setIcon(R.drawable.ic_help);
    alertDialog.setButton("Close", new DialogInterface.OnClickListener() {

    public void onClick(DialogInterface dialog, int which) {
        } });

    alertDialog.show();
    return alertDialog;
}

How to change dialog background color programmatically? has the answer.

You will have to extend the layout class to use a custom theme, as described in this tutorial: http://blog.androgames.net/10/custom-android-dialog/