* {
    box-sizing: border-box
}

html, body {
    height: 100%;
    margin: 0; 
    padding: 0;
}

body {
    background-image: url(../img/fondo.jpg);
    background-size: cover;
    background-position: center;
    font-family: sans-serif;
}

body {
    padding: 20px;
}

main {
    width: 100%;
    max-width: 960px;
    margin: 0 auto;
    background-color: rgba(255, 255, 255, .7);
    border-radius: 10px;
    padding: 20px;
    min-width: 320px;
}
.titulo{
    text-align: center;
    font-size: 3rem;
}
.formulario {
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.formulario__introducir {
    flex-basis: 75%;
    border: 0;
    padding: 5px;
}

.formulario__enviar {
    flex-basis: 25%;
    border: 0;
    background-color: gray;
    color: white;
    padding: 5px;
    cursor: pointer;
}
.formulario__enviar:hover{
    background-color: rgb(90, 90, 90);

}

.tareas {
    padding: 0;
}

.tarea {
    cursor: pointer;
    display: flex;
    justify-content: space-between;
    padding: 5px;
    margin-bottom: 5px;
}

.tarea:last-of-type {
    margin-bottom: 0;
}

.tarea__nombre {
    flex-basis: 75%;
}

.tarea--completada {
    text-decoration: line-through;
    opacity: .6;
    background-color: lightgreen;
}

@media screen and (max-width: 600px) {
    .formulario {
        flex-wrap: wrap;
    }

    .formulario > * {
        flex-basis: 100%;
    }
}