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-area.css

@@ -1,21 +1,15 @@
 @import "reset-to-defaults.css";
 
-GtkWindow {
+window {
   background-color: rgb(0,255,0);
 }
 
-GtkButton {
+button {
   border-width: 5px;
   border-style: solid;
   border-color: rgba(255,0,0,0.6);
   padding: 10px;
   background-color: rgb(0,0,255);
-
-  /* Make sure children of button are only offset by padding */
-  -GtkButton-default-border: 0;
-  -GtkButton-inner-border: 0;
-  -GtkWidget-focus-line-width: 0;
-  -GtkWidget-focus-padding: 0;
 }
 
 #button1 {