I just noticed that the borders of the Button and the ImageButton look different. The ImageButton is totally flat. I cannot explain it and I would like to have the ImageButton as "3d-ish" as the default Button. Any ideas are welcome!
This is my xml-code
<LinearLayout
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center">
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Power"
        android:id="@+id/button1"
        android:layout_gravity="center"
        android:layout_weight="0"
        android:layout_centerInParent="true" />
    <ImageButton
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:src="@drawable/f_light_on"
        android:id="@+id/button4"
        android:layout_gravity="center"
        android:layout_weight="0"
        android:layout_centerInParent="true" />
</LinearLayout>
And this is how it looks on my Lollipop tablet:

Thanks in advance! Thomas