Fix for button hover colors

fixes hover color per issue #5451

Author: Richard LaFranchi

scss/foundation/components/_buttons.scss

@@ -133,9 +133,9 @@ $button-disabled-opacity: 0.7 !default;
     $bg-lightness: lightness($bg);
 
     background-color: $bg;
-    border-color: $bc;
+    border-color: scale-color($bg, $lightness: $button-function-factor);
     &:hover,
-    &:focus { background-color: $bh; }
+    &:focus { background-color: scale-color($bg, $lightness: $button-function-factor); }
 
     // We control the text color for you based on the background color.
     @if $bg-lightness > 70% {