fix(mixins.scss): inject SassScript value to CSS variables to fix libsass 3.5.0

[VdustR]

http://sass-lang.com/documentation/file.SASS_CHANGELOG.html#3_5_0__12_July_2017_

The way CSS variables are handled has changed to better correspond to the CSS spec. They no longer allow arbitrary SassScript in their values; instead, almost all text in the property values will be passed through unchanged to CSS. The only exception is #{}, which will inject a SassScript value as before.

fix #1619