fix(Core): inject SassScript value to CSS variables to fix libsass 3.5.0 (#1620)

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

fix #1619

Author: VdustR

src/theme/mixins.scss

@@ -47,7 +47,7 @@
 
   :root {
     @each $type, $value in $palette {
-      --md-theme-#{$name}-#{$type}: $value
+      --md-theme-#{$name}-#{$type}: #{$value}
     }
   }