Add light/dark color functions

sass/utilities/functions.sass

@@ -65,3 +65,19 @@
     @return rgba(#000, 0.7)
   @else
     @return #fff
+
+@function findLightColor($color)
+  @if type-of($color) == 'color'
+    $l: 96%
+    @if lightness($color) > 96%
+      $l: lightness($color)
+    @return change-color($color, $lightness: $l)
+  @return $background
+
+@function findDarkColor($color)
+  @if type-of($color) == 'color'
+    $l: 29%
+    @if lightness($color) < 29%
+      $l: lightness($color)
+    @return change-color($color, $lightness: $l)
+  @return $text-strong

sass/utilities/initial-variables.sass

@@ -16,7 +16,7 @@ $white-bis:    hsl(0, 0%, 98%) !default
 $white:        hsl(0, 0%, 100%) !default
 
 $orange:       hsl(14,  100%, 53%) !default
-$yellow:       hsl(48,  100%, 53%) !default
+$yellow:       hsl(48,  100%, 67%) !default
 $green:        hsl(141, 53%,  53%) !default
 $turquoise:    hsl(171, 100%, 41%) !default
 $cyan:         hsl(204, 71%,  53%) !default