Fix wrong usage of color identifiers.

src/style/_colors.sass

@@ -23,9 +23,12 @@
             color: get-color($color-name, $i)
 
 // Add black and white.
-@each $color in black, white
-    .bg-#{$color}
-        background-color: $color
-    .fg-#{$color}
-        color: $color
+    .bg-black
+        background-color: black
+    .fg-black
+        color: black
+    .bg-white
+        background-color: white
+    .fg-white
+        color: white