[gtk+] reftests: Make separator-size test pass in HighContrastInverse

[Matthias Clasen <matthiasc src gnome org>]

commit 34467600607c86e70aca6cd2fee3abceb703f494
Author: Matthias Clasen <mclasen redhat com>
Date:   Sun Jan 31 16:53:53 2016 -0500

    reftests: Make separator-size test pass in HighContrastInverse
    
    After neutralizing the stack background, the notebook itself
    was peeking through.

 testsuite/reftests/separator-size.css |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/testsuite/reftests/separator-size.css b/testsuite/reftests/separator-size.css
index 2dff874..1463c97 100644
--- a/testsuite/reftests/separator-size.css
+++ b/testsuite/reftests/separator-size.css
@@ -1,3 +1,4 @@
+notebook,
 notebook stack {
   background: none;
 }