Button background as transparent

I have a button. When I press the button I have to make text as bold otherwise normal. So I wrote styles for bold & normal.

<style name="textbold" parent="@android:style/TextAppearance">
<item name="android:layout_width">wrap_content</item>
<item name="android:layout_height">wrap_content</item>
<item name="android:textStyle">bold</item>
</style>
<style name="textregular" parent="@android:style/TextAppearance">
<item name="android:layout_width">wrap_content</item>
<item name="android:layout_height">wrap_content</item>
<item name="android:textStyle">normal</item>
</style>

Now I have a button_states.xml as:

<?xml version="1.0" encoding="utf-8"?>

<selector xmlns:android="http://schemas.android.com/apk/res/android">
<item android:state_focused="true" android:state_pressed="true"
    style="@style/textbold" />
<item android:state_focused="false" android:state_pressed="true"
    style="@style/textregular" />

<item style="@style/textregular" />
</selector> 

In my layout for this button, I have to make the background as transparent too…How will I do it? My layout code is :

<Button android:id="@+id/Btn" android:background="@drawable/button_states" />

How will I include background as transparent in my style?