Go to values/style or theme and paste this code:

<resources xmlns:tools="http://schemas.android.com/tools">
    <!-- Base application theme. -->
    <style name="Theme.BookApp" parent="Theme.MaterialComponents.DayNight.DarkActionBar">
        <!-- Primary brand color. -->
        <item name="colorPrimary">@color/purple_500</item>
        <item name="colorPrimaryVariant">@color/purple_700</item>
        <item name="colorOnPrimary">@color/white</item>
        <!-- Secondary brand color. -->
        <item name="colorSecondary">@color/teal_200</item>
        <item name="colorSecondaryVariant">@color/teal_700</item>
        <item name="colorOnSecondary">@color/black</item>
        <!-- Status bar color. -->
        <item name="android:statusBarColor" tools:targetApi="l">?attr/colorPrimaryVariant</item>
        <!-- Customize your theme here. -->
    </style>
    
    <!-- style niche jeta dekha jacce eta dilei hobe .. karon uporer likha gulo already style er moddhe thakbei .. ba ei page e thakbei -->
    <style name="splashscreen" parent="Theme.MaterialComponents.DayNight.NoActionBar">

    </style>
</resources>


Android Manifest.xml code:


<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.bookapp">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.BookApp">
        
        <!-- eikhane splashscreen activity ta add korte hobe tar age obossoi splash screen ekti activity creat krte hobe .-->
        <activity android:name=".splashscreen"
            android:theme="@style/splashscreen"
            android:noHistory="true"
            >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>

        </activity>
        <!-- upore jei comment ase sei comment theke ei comment porjonto notun code-->
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

    </application>

</manifest>


before the manfiest code submit ...(you need to creat splashscreen activity)


splashscreen.kt

package com.example.bookapp

import android.content.Intent
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import java.lang.Exception

class splashscreen : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_splashscreen)

        val background = object : Thread() {
            override fun run() {
                try {
                    Thread.sleep(5000)
                    val intent = Intent(baseContext, MainActivity::class.java)
                    startActivity(intent)
                } catch (e: Exception) {
                    e.printStackTrace()
                }
            }

        }
        background.start()


    }
}


layout/activity_splashscreen.xml code here:


package com.example.bookapp

import android.content.Intent
import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import java.lang.Exception

class splashscreen : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_splashscreen)

        val background = object : Thread() {
            override fun run() {
                try {
                    Thread.sleep(5000)
                    val intent = Intent(baseContext, MainActivity::class.java)
                    startActivity(intent)
                } catch (e: Exception) {
                    e.printStackTrace()
                }
            }

        }
        background.start()


    }
}