Transparent

From Bootstrap - Demo Wiki
Color
Name Primary
Description A transparent background that will take on the color of its container.
Provider Skin, BootstrapComponents
Prefixes bg
Reference getbootstrap.com - color

Background

Code
<div class="p-3 bg-secondary">
  <div class="p-3 text-light bg-transparent border border-warning" >
This is div with light text over a transparent background and a warning border inside a secondary colored background!
  </div>
</div>
Result

This is div with light text over a transparent background and a warning border inside a secondary colored background!