fix(button): add transparent background for clear/outline windows buttons

references #6202

Author: brandyscarney

ionic/components/button/button.wp.scss

@@ -171,6 +171,7 @@ $button-wp-fab-border-radius:                         50% !default;
     $fg-color: color-shade($color-base, 5%);
     border-color: $fg-color;
     color: $fg-color;
+    background-color: transparent;
 
     &:hover:not(.disable-hover) {
       border-color: $fg-color;
@@ -210,6 +211,7 @@ $button-wp-fab-border-radius:                         50% !default;
   .button-clear-#{$color-name} {
     $fg-color: $color-base;
     color: $fg-color;
+    background-color: transparent;    
 
     &.activated {
       background-color: $button-wp-clear-background-color-activated;