[gnome-builder] style: tweak insensitive buttons in stack header

[Christian Hergert <chergert src gnome org>]

commit b8ffda572f3870d67cd93207410d0ef36cfd512b
Author: Christian Hergert <chergert redhat com>
Date:   Sun Sep 3 12:30:23 2017 -0700

    style: tweak insensitive buttons in stack header

 data/themes/shared/shared-layout.css |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/data/themes/shared/shared-layout.css b/data/themes/shared/shared-layout.css
index b7b8eae..4b57413 100644
--- a/data/themes/shared/shared-layout.css
+++ b/data/themes/shared/shared-layout.css
@@ -9,6 +9,9 @@ idelayoutstackheader button {
   padding: 0;
   margin: 0;
 }
+idelayoutstackheader button:disabled {
+  color: alpha(currentColor, 0.25);
+}
 idelayoutstackheader button:not(:disabled) image {
   opacity: 0.55;
 }