hi check this http://developer.android.com/guide/topics/ui/themes.html
in this define the style and theme for the application.
Creating custom style and theme and set for single/multiple control with different style and theme
this is my main.xml file
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="fill_parent"
android:layout_height="fill_parent" android:id="@+id/linear">
<TextView android:layout_width="fill_parent"
android:layout_height="wrap_content" android:text="@string/hello" style="@style/CodeFont"/>
<Button android:id="@+id/btn" android:layout_width="200dp"
android:layout_height="wrap_content" android:text="Click" style="@style/CodeFont"/>
<ImageView android:id="@+id/img_preview"
android:layout_width="fill_parent" android:layout_height="wrap_content" />
</LinearLayout>
and this is my style.xml file
<?xml version="1.0" encoding="utf-8"?>
<resources>
<style name="CodeFont" parent="@android:style/TextAppearance.Medium">
<item name="android:layout_width">fill_parent</item>
<item name="android:layout_height">wrap_content</item>
<item name="android:textColor">#00FF00</item>
<item name="android:typeface">monospace</item>
</style>
</resources>