Алексей Уколов

&__active
    &.btn__negative
      background-color red
      color white

    &.btn__positive
      background-color green
      color white

CSS 2.1 properties (well-supported subset): background-image
These properties are supported in effectively all browsers (since IE6+, Firefox 2+, Chrome 1+ etc)
Includes the following:
...
background-image
...