Button

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@drawable/baackground"
    tools:context=".MainActivity" >

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_alignParentRight="true"
        android:layout_alignParentTop="true"
        android:background="@android:color/black"
        android:textColor="@android:color/white"
         
        android:text="Select Photo sizes" />

</RelativeLayout>

in activity.xml use the following code:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"

    xmlns:tools="http://schemas.android.com/tools"

    android:layout_width="match_parent"

    android:layout_height="match_parent"

    android:background="@drawable/baackground"

  

    tools:context=".MainActivity" >

    <Button

        android:id="@+id/button1"

        style="@style/Custom"

        android:layout_width="wrap_content"

        android:layout_height="wrap_content"

        android:layout_alignParentLeft="true"

        android:layout_alignParentTop="true"

        android:layout_marginLeft="28dp"

        android:text="Button" />

    

    

</RelativeLayout>

now make a folder-drawable in res folder where we create a selector.xml file and following code is used...

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

<shape xmlns:android="http://schemas.android.com/apk/res/android"

    android:shape="oval"  >

    <solid android:color="#f00"/>

    <size android:width="55dip"

        android:height="55dip"/>

</shape>

now,finally inside the values folder where we can find the styles.xml file,here we code our program like:

<resources xmlns:android="http://schemas.android.com/apk/res/android">

    <!--

        Base application theme, dependent on API level. This theme is replaced

        by AppBaseTheme from res/values-vXX/styles.xml on newer devices.

    -->

    <style name="AppBaseTheme" parent="android:Theme.Light">

        <!--

            Theme customizations available in newer API levels can go in

            res/values-vXX/styles.xml, while customizations related to

            backward-compatibility can go here.

        -->

    </style>

    <!-- Application theme. -->

    <style name="AppTheme" parent="AppBaseTheme">

        <!-- All customizations that are NOT specific to a particular API-level can go here. -->

    </style>

    <style name="Custom" parent="@android:style/Widget.Button">

        <!-- All customizations that are NOT specific to a particular API-level can go here. -->

       <item name="android:background">@drawable/selector</item>

    </style>

</resources>

As we look above i have called the selector in item name of styles.xml..