The styling of components like that can vary from device to device. So, if you want it to stay the same on all devices you can specify a background for it. Something like;
<Spinner
android:id="@+id/spinPlayer"
android:layout_width="128dp"
android:layout_height="wrap_content"
android:layout_gravity="top"
android:background="#FFFFFF"
android:layout_marginTop="20dp" />
The example above will make the background entirely white. If you want a more robust background like the one you linked in some comments, then you'll need to create an image file, store it in your drawable folder and refer to it like so;
android:background="@drawable\your_spinner_background_image"