Original card

<div class="card">
    <div class="card-header">
        Featured
    </div>
    <div class="card-block">
        <h4 class="card-title">Special title treatment</h4>
        <p class="card-text">With supporting text below as a natural lead-in to additional content.</p>
        <button class="btn btn-primary">Go somewhere</button>
    </div>
</div>

Gray card (twbs3 well)

<div class="sc-white-bg">
    <div class="card card-well">
        <div class="card-block">
            <h4><span class="digit-circle">1</span> Some title</h4>
            Lorem ipsum dolor sit amet, consectetur adipiscing elit. Quisque
            suscipit porttitor urna at molestie. Etiam nec porttitor nunc, ac molestie nisl. Ut
            velit eros, tempor eget fermentum porta, condimentum a urna. Praesent eget fermentum
            ex. Duis bibendum aliquam nibh sed consectetur. In a scelerisque orci. Sed eu gravida
            enim, eget feugiat turpis. Morbi ac pharetra lacus, non tincidunt justo. Nullam
            accumsan ac ex ac eleifend. Maecenas viverra eget velit et commodo. Sed a laoreet
            lorem, at tempor mi. Suspendisse potenti. Nulla facilisi.
        </div>
    </div>
</div>

Card with section header

<div class="card">
    <div class="card-block">
        <h2>Some title</h2>
        <p>Lorem ipsum dolor sit amet...</p>
        <p>Lorem ipsum dolor sit amet...</p>
    </div>
</div>