/* Core Styles */
body {
    margin: 0;
    font-family: Arial, Helvetica, sans-serif;
    background: #333;
    font-size: 1.1em;
    color: #fff;
    line-height: 1.5;
    text-align: center;
}

img {
    display: block; 
    width: 100%;
    height: auto;   
}

h1, h2, h3 {
    margin: 0;
    padding: 1em 0;
}

p {
  margin: 0;
  padding: 1em 0;

}

.btn {
 display: inline-block;
 background: #333;
 color: #fff;
 text-decoration: none; 
 padding: 1em 2em;
 border: 1px solid #666;
 margin: .5em 0;
}

.btn:hover {
    background: #eaeaea;
    color: #333;
}

/* Header Showcase */
#showcase {
    min-height: 450px;
    color: #fff;
    text-align: center;
}

#showcase .bg-image {
    position: absolute;
    background: #333 url('https://cdn.pixabay.com/photo/2016/11/30/20/58/programming-1873854_1280.png');
    background-position: center;
    background-repeat: no-repeat;   
    width: 100%;
    /* 和Showcase一样 */
    height: 450px;
    z-index: -1;
    opacity: 0.4;
}

#showcase h1 {
    padding-top: 100px;
    padding-bottom: 0;
}

#showcase .content-wrap,
#section-a .content-wrap {
    padding: 0 1.5em;
}

/* Main Area */
/* Section A */
#section-a {
    background: #eaeaea;
    color: #333;
    padding-bottom: 2em;
}

/* Section B */
#section-b {
    padding: 2em 1em 1em;
}

#section-b ul {
    list-style: none;
    margin: 0;
    padding: 0;
}

#section-b li {
    margin-bottom: 1em;
    background: #fff;
    color: #333;
}

.card-content {
    padding: 1.5em;
}

/* Section C */
#section-c {
    background: #fff;
    color: #333;
    padding: 2em;
}

/* Section D */
#section-d .box {
    padding: 2em;
    color: #fff;
}

#section-d .box:first-child {
    background: #2690d4;
}

/* Footer */
#main-footer {
    padding: 2em;
    background: #000;
    color: #fff;
    text-align: center;
}

#main-footer a {
    color: #2690d4;
    text-decoration: none;
}

/* Media Queries  */
@media(min-width: 700px) {
    .grid {
        display: grid;
        grid-template-columns: 1fr repeat(2, minmax(auto, 25em)) 1fr;
    }

    #section-a .content-text {
        columns: 2;
        column-gap: 2em;
    }

    #section-a .content-text p {
        /* 两列文字水平放置 */
        padding-top: 0;
    }

    .content-wrap, 
    #section-b ul {
        grid-column: 2/4;
    }

    .box, #main-footer div {
        grid-column: span 2;

    }

    #section-b ul {
        display: flex;
        /* 弹性 */
        justify-content: space-around;
    }

    #section-b li {
        width: 31%;
    }
}