styles.css

html {
    font-size: 10px;
}

body, input, button {
    font-family: 'Helvetica Neue', Helvetica, Roboto, Arial, sans-serif;
    font-size: 1.8rem; 
}

body {
    margin: 0;
}

form {
    margin: 0;
    position: relative;
}

form input {
    background: transparent;
    border: 0;
    border-bottom: 0.1rem dashed black;
    font-weight: 500;
    line-height: 5.2rem;
    padding-bottom: 0.1rem;
    padding-right: 2rem;
    padding-top: 0.6rem;
    width: 100%;
}

form input:focus {
    border-bottom: 0.1rem solid black;
    outline: none;
}

form button {
    background: url(assets/add.svg) center center;
    border: 0.1rem solid transparent;
    cursor: pointer;
    height: 4rem;
    position: absolute;
    right: -1.2rem;
    top: 1rem;
    width: 4rem;
}

form button:focus {
    border: 0.1rem solid black;
    border-radius: 2rem;
    outline: none;
}

ul {
    margin: 0;
    padding: 3rem 0;
}

ul li {
    line-height: 2rem;
    list-style: none;
    margin: 0;
    padding: 1.4rem 0 0.6rem 4rem;
    position: relative;
}

ul li button {
    background: url(assets/done.svg) center center;
    border: 0.1rem solid transparent;
    cursor: pointer;
    height: 4rem;
    left: -0.8rem;
    position: absolute;
    top: 0.2rem;
    width: 4rem;
}

ul li button:focus {
    border: 0.1rem solid black;
    border-radius: 2rem;
    outline: none;
}

ul li.pending span {
    font-weight: 500;
}

ul li.done button {
    display: none;
}

ul li.done span {
    color: rgba(0,0,0,0.5);
    text-decoration: line-through;
}

@media screen and (max-width: 600px) {
    main {
        padding: 0 2.0rem;
    }
} 

@media screen and (min-width: 601px) {
    main {
        padding: 0;
        width: 50rem;
        margin: 0 auto;
    }
} 

@media screen and (min-width: 1201px) {
    html {
        font-size: 15px;
    }
}