java.lang.IllegalStateException: You need to use a Theme.AppCompat theme (or descendant) with this activity

I am trying to make an alertdialog but it is giving error:

06-08 13:04:38.318 22518-22518/realsysten.com.br.sigarestaurante E/AndroidRuntime: FATAL EXCEPTION: main
                                                                               Process: realsysten.com.br.sigarestaurante, PID: 22518
                                                                               java.lang.IllegalStateException: You need to use a Theme.AppCompat theme (or descendant) with this activity.
                                                                                   at android.support.v7.app.AppCompatDelegateImplV7.createSubDecor(AppCompatDelegateImplV7.java:340)
                                                                                   at android.support.v7.app.AppCompatDelegateImplV7.ensureSubDecor(AppCompatDelegateImplV7.java:309)
                                                                                   at android.support.v7.app.AppCompatDelegateImplV7.setContentView(AppCompatDelegateImplV7.java:273)
                                                                                   at android.support.v7.app.AppCompatDialog.setContentView(AppCompatDialog.java:80)
                                                                                   at android.support.v7.app.AlertController.installContent(AlertController.java:214)
                                                                                   at android.support.v7.app.AlertDialog.onCreate(AlertDialog.java:256)
                                                                                   at android.app.Dialog.dispatchOnCreate(Dialog.java:394)
                                                                                   at android.app.Dialog.show(Dialog.java:295)
                                                                                   at realsysten.com.br.sigarestaurante.AlterarAtendenteActivity$3.onClick(AlterarAtendenteActivity.java:100)
                                                                                   at android.view.View.performClick(View.java:5198)
                                                                                   at android.view.View$PerformClick.run(View.java:21147)
                                                                                   at android.os.Handler.handleCallback(Handler.java:739)
                                                                                   at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                                   at android.os.Looper.loop(Looper.java:148)
                                                                                   at android.app.ActivityThread.main(ActivityThread.java:5417)
                                                                                   at java.lang.reflect.Method.invoke(Native Method)
                                                                                   at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                                                   at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

Class with dialog:

public class AlterarAtendenteActivity extends AppCompatActivity {

    EditText nome;
    EditText senha;
    Button confirmaAltera;
    Button cancelaAltera;
    Button deletaAt;
    Cursor cursor;
    BancoController crud;
    String codigo;

    private AlertDialog alerta;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.alterar_atendente);

        codigo = this.getIntent().getStringExtra("codigo");
        crud = new BancoController(getBaseContext());

        nome = (EditText) findViewById(R.id.edtAltNome);
        senha = (EditText) findViewById(R.id.edtAltSenha);

        confirmaAltera = (Button) findViewById(R.id.btnAlterarAtendente);
        cancelaAltera = (Button) findViewById(R.id.btnCancelarAltAtend);
        deletaAt = (Button) findViewById(R.id.btnDeletarAtendente);

        cursor = crud.carregarAtendenteId(Integer.parseInt(codigo));
        nome.setText(cursor.getString(cursor.getColumnIndexOrThrow(CriaBanco.getAtNome())));
        senha.setText(cursor.getString(cursor.getColumnIndexOrThrow(CriaBanco.getAtSenha())));

        confirmaAltera.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                crud.alteraAtendente(Integer.parseInt(codigo), nome.getText().toString(),
                        senha.getText().toString());
                Intent i = new Intent(AlterarAtendenteActivity.this, LoginAtendActivity.class);
                LoginAtendActivity.op = 1;
                startActivity(i);
                finish();
            }
        });

        cancelaAltera.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent i = new Intent(AlterarAtendenteActivity.this, OpcoesActivity.class);
                LoginAtendActivity.op = 1;
                startActivity(i);
                finish();
            }
        });

        deletaAt.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                AlertDialog.Builder builder = new AlertDialog.Builder(getBaseContext());
                builder.setTitle("Alerta");
                builder.setMessage("Deseja realmente excluir o atendente: " + nome);
                builder.setPositiveButton("Sim", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialog, int which) {

                        crud.deletaAtendente(Integer.parseInt(codigo));
                        Intent i = new Intent(AlterarAtendenteActivity.this, LoginAtendActivity.class);
                        LoginAtendActivity.op = 1;
                        Toast.makeText(AlterarAtendenteActivity.this, "Atendente deletado",
                                Toast.LENGTH_LONG);
                        startActivity(i);
                        finish();
                    }
                });

                builder.setNegativeButton("Não", new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialog, int which) {
                        Toast.makeText(AlterarAtendenteActivity.this, "Atendente não deletado",
                                Toast.LENGTH_LONG);
                    }
                });

                alerta = builder.create();
                alerta.show();

            }
        });

    }
}

Can anyone help me?