style.css

:root{
    --headerColor: rgb(41, 50, 60);
    --backgroundColor: rgb(36, 39, 48);
    --elementColor: rgb(106, 116, 133);
}

html, body {
    height: 100%;
    margin: 0;
    background-color: var(--backgroundColor);
}

.box {
    display: flex;
    flex-flow: column;
    height: 100%;
}

iframe.content {
    border-style: none;
    flex: 1 1 auto;
    background-color: var(--backgroundColor);
}

.header {
    padding: 15px 15px;
    flex: 0 1 auto;
    background-color: var(--headerColor);
}

button, select {
    background-color: var(--elementColor);
    border-width: 0;
    color: white;
    cursor: pointer;
    padding: 10px;
    margin-right: 10px;
    font-size: 20px;
    border-radius: 4px;
}

button:hover, select:hover {
  box-shadow: 0 12px 16px 0 rgba(0,0,0,0.24), 0 17px 50px 0 rgba(0,0,0,0.19);
  transition-duration: 0.5s;
}

#GitHubReference {
    position: absolute;
    right: 15px;
    margin: 3px;
    color: lightgray;
    font-size: 36px;
    cursor: pointer;
}