6.2.2 #permalink Overlay
Put a dark overlay over an image. If this div's parent is an a, the overlay will be lightened on hover.
Example
Markup
<a href="#" class="grid-3">
<div class="img-overlay">
<img class=" img-cover" src="/img/banners/photoBlock4.jpg" alt="Our team loves cereal"/>
</div>
</a>
Source:
modules/06_images/image.less, line 48