Reuse of the color-yiq function

This change refers to the issue twbs#23114

scss/_functions.scss

@@ -49,17 +49,17 @@
 }
 
 // Color contrast
-@mixin color-yiq($color) {
+@mixin color-yiq($color, $prop: 'color') {
   $r: red($color);
   $g: green($color);
   $b: blue($color);
 
   $yiq: (($r * 299) + ($g * 587) + ($b * 114)) / 1000;
 
   @if ($yiq >= 150) {
-    color: #111;
+    #{$prop}: #111;
   } @else {
-    color: #fff;
+    #{$prop}: #fff;
   }
 }