style-clean.css

* {
    box-sizing: border-box;
}

body {
    background-image: url('http://thumbs.dreamstime.com/x/cute-friendly-cow-pattern-7279386.jpg');
    text-align: center;
}

#app {
    margin-top: 30px;
    text-align: left;
    padding: 10px;
    width: 600px;
    background-color: white;
    overflow: auto;
    margin-left: auto;
    margin-right: auto;
}

ul {
    list-style: none;
    padding-left: 0;
}
li {
    padding-left: 0;
}


.thumbnail {
    width: 50%;
    float: left;
    padding: 5px;
}
.thumbnail:nth-child(2n+1) {
    clear: both;
}

.thumbnail.selected {
    background-color: white;
    border: 1px solid silver;
}

.common {
    color: #999;
}
.common:before {
    content: " ";
}

.plus {
    color: green;
}
.plus:before {
    content: "+";
}

.minus {
    color: red;
}
.minus:before {
    content: "-";
}

.common:before, .plus:before, .minus:before {
    display: inline-block;
    min-width: 1em;
    text-align: center;
}



.thumbnail img {
  display:none;
}