Fix another reftest

Make sure the notebook in the separator-size reftest does not
peek through, by giving it a transparent background.

testsuite/reftests/Makefile.am

@@ -380,6 +380,7 @@ testdata = \
 	revealer-wrappable-contents.ui \
 	rotated-layout.ref.ui \
 	rotated-layout.ui \
+	separator-size.css \
 	separator-size.ref.ui \
 	separator-size.ui \
 	set-default-direction.ui \

testsuite/reftests/separator-size.css

@@ -0,0 +1,3 @@
+notebook stack {
+  background: none;
+}