styles.css

html, body {
    width: 100%;
    height: 100%;
    box-sizing: border-box;
    margin: 0;
    padding: 0;
    background: #333;
}


/* preview section */
.page-preview-section {
    position: relative;
    box-sizing: border-box;
    float: left;
    width: 80%;
    height: 100%;

    background: #111;
}

.page-preview-image {
    height: 100%;
    width: 100%;
    background-color: #000;
}



/* navigation or article list section */
nav {
    box-sizing: border-box;
    width: 20%;
    height: 100%;
    float:right;
    background: #333;
    color: #fff;
    overflow: scroll;
    padding: 20px;
    font-family: Arial;
    font-size: 14px;
}

nav > .btn-100 {
    width: 100%;
    box-sizing: border-box;
    padding: 1em;
    background-color: transparent;
    border: 2px solid #222;
    border-radius: 5px;
    font-size: 14px;
}

.article-list-section {
    box-sizing: border-box;
    padding: 1em 0;
    margin: 0;
}

.article-list-section > li {
    box-sizing: border-box;
    display: block;
    padding: 1em;
    width: 100%;
    border-radius: 5px;
}

.article-list-section > li:hover {
    background-color: rgba(0, 0, 0, .5);
}