[Label] Use basicBackground with background

[ColinFrick]

Instead of background the property background-color was used.

Fixes #6439

[jlukic]

The variable is indeed background

https://github.com/Semantic-Org/Semantic-UI/blob/master/src/themes/default/elements/label.variables#L106

@basicBackground: none @white;

[kevinresol]

This fix is correct.