reftests: Fix up for easy CSS changes

This is mostly search and replace ala
  GtkButton => button
  GtkWindow => window
  .button => button
or removing style properties that aren't used anymore like
  -GtkButton-default-border: 0

testsuite/reftests/background-window-transparent.css

@@ -1,5 +1,5 @@
 @import "reset-to-defaults.css";
 
-GtkWindow { background-color: lime; }
+window { background-color: lime; }
 
 #reference * { background-color: lime; }