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/gtk-icontheme-sizing.css

@@ -1,3 +1,3 @@
-GtkBox {
+box {
   background-image: -gtk-icontheme("pan-down-symbolic");
 }