【Android Studio】How to Customize Spinner

MainActivity.java

package com.example.customspinner;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Spinner;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Spinner spinner = findViewById(R.id.spinner);

        ArrayAdapter<String> adapter = new ArrayAdapter<>(
                this,
                R.layout.custom_spinner,
                getResources().getStringArray(R.array.list)
        );
        adapter.setDropDownViewResource(R.layout.custom_spinner_dropdown);
        spinner.setAdapter(adapter);

        spinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
            @Override
            public void onItemSelected(AdapterView<?> adapterView, View view, int position, long l) {
                String selectedItem = spinner.getItemAtPosition(position).toString();
                Toast.makeText(getApplicationContext(), selectedItem, Toast.LENGTH_LONG).show();
            }
            @Override
            public void onNothingSelected(AdapterView<?> adapterView) {

            }
        });
    }
}