@import url(https://fonts.googleapis.com/css?family=Roboto+Mono:700);

.header {
    font-family: 'Roboto Mono';   
}

.header h1 {
    font-size: 54px;
    text-align: center;
    margin-bottom: 0;
}

.header ul {
    display: flex;
    flex-direction: row;
    justify-content: space-between;
    padding: 0;
    max-width: 480px;
    margin-top: 0;
    margin-left: auto;
    margin-right: auto;
}

.header li {
    display: inline-block;
}

@import url(https://fonts.googleapis.com/css?family=Roboto+Mono:700);

body {
    font-family: 'Roboto Mono';   
}

.header h1 {
    font-size: 54px;
    text-align: center;
    margin-bottom: 0;
}

.header ul {
    display: flex;
    flex-direction: row;
    justify-content: space-between;
    padding: 0;
    max-width: 480px;
    margin-top: 0;
    margin-left: auto;
    margin-right: auto;
}

.header li {
    display: inline-block;
}

Move font-family declaration to body
Move font-family declaration to body
