thoughtbot/bourbon — pull request #507 by Nevraeka (2014-08-28 20:13:42+00:00)
Title: [Issue #498] Added extra optional parameter for layered transparency

Description
|-- Added optional '$transparency' variable in the event you want to
| force transparent 'background-color'. $fallback will always take the
| first color stop if it's not set in the mixin. By forcing a
| transparent background, you can do layered gradient effects.