Android NumberPicker Plus/Minus Signs Not Centered -

i created dialog numberpicker, can't center plus , minus signs. here code , image, appretiated!

<relativelayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="fill_parent" android:layout_height="wrap_content" >  <numberpicker     android:id="@+id/numberpicker1"     android:layout_width="fill_parent"     android:layout_height="wrap_content"     android:layout_alignparenttop="true"     android:layout_centerhorizontal="true"     android:layout_marginbottom="20dp"/>  <button     android:id="@+id/cancel"     android:layout_width="150dp"     android:layout_below="@+id/numberpicker1"     android:layout_height="wrap_content"     android:layout_alignparentleft="true"     android:text="cancel" />  <button     android:id="@+id/set"     android:layout_width="150dp"     android:layout_height="wrap_content"     android:layout_below="@+id/numberpicker1"     android:layout_alignparentright="true"     android:text="set" />  /relativelayout>