main.css

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

#header {
    padding: 0.5rem 0 0.5rem 0;
    
    background: #485563; /* fallback for old browsers */
	background: -webkit-linear-gradient(to left, #485563 , #29323c); /* Chrome 10-25, Safari 5.1-6 */
	background: linear-gradient(to left, #485563 , #29323c); /* W3C, IE 10+/ Edge, Firefox 16+, Chrome 26+, Opera 12+, Safari 7+ */
}

#header h1 {
    text-align: center;
    color: #dddddd;
    font-family: "Helvetica Neue", Helvetica, "Segoe UI", Arial, freesans, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
}

#content {
    margin: 2rem 1rem 1.5rem 1rem;
    font-size: 1.8rem;
}

#input {
    font-size: 1.2rem;
    display: block;
    padding: 1rem;
    /* width: calc(100% - 1rem); /** css 3 baby **/
    border-radius: 0.3rem;
}

#output {
    font-size: 1.2rem;
    border: 1px solid darkmagenta;
    border-radius: 0.2rem;
    padding: 1rem;
    flex: 1;
    overflow-y: scroll;
}

#flex-container {
    min-height: 70%;
    display: flex;
    justify-content: center;
    flex-direction: row;
}

#left-seg {
    flex-grow: 1;
}

#input {
    width: 90%;
}

#right-seg {
    flex-grow: 1;
}

#output {
    width: 90%;
}

#copyAnswer {
    color: #485563;
    font-size: 1rem;
}

#buttons-container {
    display: flex;
    flex-direction: row;
    justify-content: center;
}

@media all and (max-width: 768px) {
    #flex-container {
        display: block;
    }
    #output {
        margin-top: 1rem;
    }
}