High Contrast: distinct insensitive button

Tone down insensitive buttons.

https://bugzilla.gnome.org/show_bug.cgi?id=746490

gtk/theme/HighContrast/_colors.scss

@@ -27,7 +27,7 @@ $osd_borders_color: transparentize(#fff,0.8);
 //insensitive state derived colors
 $insensitive_fg_color: mix($fg_color, $bg_color, 50%);
 $insensitive_bg_color: mix($bg_color, $base_color, 60%);
-$insensitive_borders_color: $borders_color;
+$insensitive_borders_color: mix($borders_color, $base_color, 50%);
 
 //colors for the backdrop state, derived from the main colors.
 $backdrop_base_color: $base_color;

gtk/theme/HighContrast/_drawing.scss

@@ -206,7 +206,7 @@
   //
     color: $insensitive_fg_color;
     background-color: $insensitive_bg_color;
-    border-color: $backdrop_borders_color;
+    border-color: $insensitive_borders_color;
     background-image: none;
   }
 
@@ -215,7 +215,7 @@
   // backdrop insensitive pushed button
   //
     color: $insensitive_fg_color;
-    border-color: $backdrop_borders_color;
+    border-color: $insensitive_borders_color;
     background-image: none;
     background-color: mix($backdrop_borders_color,$backdrop_bg_color, 8%);
   }