Black outline for text in boxes

How To
1

How could we add the black outline as you have in the boxes for the texts below the links: https://drupal-tools.web.cern.ch/

Thank you in advance for your reply.

2

Hello Salome,

The boxes you refer are 3 Article Boxes in a section. The black outline appears by default if you add an image as background.

Konstantinos

3

Thank you!