jetpack compose - Box rounded corners

MainActivity.kt

package com.cfsuman.jetpackcompose

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import androidx.activity.compose.setContent
import androidx.compose.foundation.background
import androidx.compose.foundation.layout.*
import androidx.compose.foundation.shape.RoundedCornerShape
import androidx.compose.material.*
import androidx.compose.runtime.*
import androidx.compose.ui.Modifier
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.tooling.preview.Preview
import androidx.compose.ui.unit.dp

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        setContent {
            GetScaffold()
        }
    }


    @Composable
    fun GetScaffold(){
        Scaffold(
            topBar = {TopAppBar(
                title = {Text(
                    "Compose - Box rounded corners",
                    color = Color.White)},
                backgroundColor = Color(0xFF333399)) },
            content = { MainContent()}
        )
    }


    @Composable
    fun MainContent(){
        Row(Modifier.padding(32.dp).background(Color(0xFFF0FFFF))){
            Box(
                Modifier.size(300.dp,150.dp)
                    .background(
                        color = Color(0xFFA52A2A),
                        shape = RoundedCornerShape(15.dp)
                    )
            )
        }
    }


    @Preview
    @Composable
    fun ComposablePreview(){
        //GetScaffold()
    }
}