如果mainActivity这种导的包是Activity如:
public class MainActivity extends Activity {
则在manifects的activity标签中写
android:theme="@android:style/Theme.Back.NoTitleBar">
如果mainActivity这种导的包是AppCompatActivity如:
public class MainActivity extends AppCompatActivity
则在manifects的activity标签中写
android:theme="@style/Theme.AppCompat.Light.NoActionBar"