.box{
border:1px solid #ccc;
font-size:12px;
background:#f1f1f1;
padding:10px;
}

<div class="box">this is a gray box</div>

.box-gray,
.box-green{
border:1px solid #ccc;
font-size:12px;
padding:10px;
}
.box-gray{background:#f1f1f1}
.box-green{background:#66ff66}

<div class="box-gray">this is a gray box</div>
<div class="box-green">this is a green box</div>

.fs-12{font-size:12px}
.fs-14{font-size:14px}
.pd-10{padding:10px}
.pd-20{padding:20px}

.box{
border:1px solid #ccc;
}
.box.gray{background:#f1f1f1}
.box.green{background:#66ff66}

<div class="box gray fs-12 pd-20">this a gray fontsize12px padding20px box</div>
<div class="box green fs-14 pd-10">this a gray fontsize14px padding10px box</div>