求高手们看下这个界面怎么居中，代码全部放上来，恳求指导！

求高手们看下这个界面如何居中，代码全部放上来，恳求指导！！！
刚开始学android，好多不会，照着书做例子，一个数独游戏的开始界面，前几天不知道怎么搞，最后七七八八的能运行了，可是现在这个界面不居中，想问下怎么居中。
这个是运行出来的图


这个是res/layout/activity.xml文件

<?xml version="1.0" encoding="utf-8"?>
    <LinearLayout
        android:background="@color/background"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:orientation="horizontal"
        xmlns:android="http://schemas.android.com/apk/res/android">
        <LinearLayout
            android:orientation="vertical"
            android:layout_width="wrap_content"
        android:layout_height="fill_parent"
        android:layout_gravity="center">
            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="center"
                android:textSize="25sp"
                android:textColor="#7ba6df"
                android:text="@string/activity_main_title"/>
            <Button
                android:id="@+id/continue_button"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:text="@string/continue_lable"/>
            <Button
                android:id="@+id/new_button"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:text="@string/new_game_lable"/>
            <Button
                android:id="@+id/about_button"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:text="@string/about_lable"/> 
 
            <Button
                android:id="@+id/exit_button"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:text="@string/exit_lable"/>
    </LinearLayout>
    </LinearLayout>

 
                android:textSize="25sp"
                android:textColor="#7ba6df"
                android:text="@string/activity_main_title"/>
            <Button
                android:id="@+id/continue_button"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:text="@string/continue_lable"/>
            <Button
                android:id="@+id/new_button"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:text="@string/new_game_lable"/>
            <Button
                android:id="@+id/about_button"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:text="@string/about_lable"/>
            <Button
                android:id="@+id/exit_button"
                android:layout_width="fill_parent"
                android:layout_height="wrap_content"
                android:text="@string/exit_lable"/>
        </LinearLayout>
    </LinearLayout>