Change Theme Programmatically Android

Themes and Styles on Android allow us to separate the details of our app design from the UI structure and behavior, Similar to Stylesheets in web design. Download source code from here

activity_main.xml:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <RelativeLayout
        android:id="@+id/rl_header"
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:background="@color/colorPrimary"></RelativeLayout>

    <Button
        android:id="@+id/btn_red"
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:layout_marginLeft="10dp"
        android:layout_marginTop="10dp"
        android:layout_marginRight="10dp"
        android:text="Red" />

    <Button
        android:id="@+id/btn_yellow"
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:layout_marginLeft="10dp"
        android:layout_marginTop="10dp"
        android:layout_marginRight="10dp"
        android:text="Yellow" />


    <Button
        android:id="@+id/btn_green"
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:layout_marginLeft="10dp"
        android:layout_marginTop="10dp"
        android:layout_marginRight="10dp"
        android:text="Green" />

    <Button
        android:id="@+id/btn_purple"
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:layout_marginLeft="10dp"
        android:layout_marginTop="10dp"
        android:layout_marginRight="10dp"
        android:text="Purple" />

</LinearLayout>

MainActivity.kt:

package changetheme.deepshikha.com.changetheme

import android.graphics.Color
import android.os.Build
import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.view.View
import android.view.WindowManager
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity(), View.OnClickListener {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        listener()
        init()
    }

    private fun init() {

    }

    private fun listener() {
        btn_red.setOnClickListener(this)
        btn_yellow.setOnClickListener(this)
        btn_green.setOnClickListener(this)
        btn_purple.setOnClickListener(this)
    }


    override fun onClick(v: View?) {
        when (v?.id) {
            R.id.btn_red -> {
                fn_changethemecolor(resources.getColor(R.color.theme_red), resources.getColor(R.color.red))
            }

            R.id.btn_yellow -> {
                fn_changethemecolor(resources.getColor(R.color.theme_yellow), resources.getColor(R.color.yellow))
            }

            R.id.btn_green -> {
                fn_changethemecolor(resources.getColor(R.color.theme_green), resources.getColor(R.color.green))
            }

            R.id.btn_purple -> {
                fn_changethemecolor(resources.getColor(R.color.theme_purple), resources.getColor(R.color.purple))
            }
        }
    }


    /**
     * Method for change theme color
     */
    fun fn_changethemecolor(color: Int, header_color: Int) {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
            val window = window
            window.addFlags(WindowManager.LayoutParams.FLAG_DRAWS_SYSTEM_BAR_BACKGROUNDS)
            window.statusBarColor = color

        }
        rl_header.setBackgroundColor(header_color)

    }
}