<Button variant="contained" id="btnEntrar" style={{
                                backgroundColor: '#67BF59',
                                fontSize:25,
                                fontWeight: 'bold'
                            }}
                            color="primary" onClick={() => {
                            alert('yeah button');
}}>Entrar</Button>
<p style={{zIndex:-1}}>aeaeaesa</p>

//css


#btnEntrar {
    padding:20px 60px;
    outline: none;
    background-color: #27ae60;
    border: none;
    border-radius:5px;
    width:220px;
    font-family: 'Roboto';
    transition: all 0.5;
}