What result in black background in the zone of div#secondary?

For theme –twentyfourteen.
What result in black background in the zone of div#secondary?
From the computed in firebug,the background color of div#secondary is transparent,why it is displayed as black?
div#secondary’s background color to discuss in twentyfourteen theme

• This topic was modified 7 years, 6 months ago by elearn2014.
• This topic was modified 7 years, 6 months ago by elearn2014.