Administration: Use #fff instead of white

Cleans up a few places where the CSS coding standards were not being followed.

Fixes #37576.
Props Presskopp, johnpgreen, netweb



git-svn-id: https://develop.svn.wordpress.org/trunk@38501 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/css/colors/_mixins.scss

@@ -2,7 +2,7 @@
  * Button mixin- creates 3d-ish button effect with correct
  * highlights/shadows, based on a base color.
  */
-@mixin button( $button-color, $text-color: white ) {
+@mixin button( $button-color, $text-color: #fff ) {
 	background: $button-color;
 	border-color: darken( $button-color, 10% ) darken( $button-color, 15% ) darken( $button-color, 15% );
 	color: $text-color;

src/wp-admin/css/common.css

@@ -680,7 +680,7 @@ ul.striped > :nth-child(odd),
 }
 
 .wp-ui-highlight {
-	color: white;
+	color: #fff;
 	background-color: #1e8cbe;
 }
 .wp-ui-text-highlight {