themes: Actually apply the frame.flat style class

The docs say that this class should be put on the frame node, and that’s
all we can do from C code, but the CSS was selecting on the border node.
The result was that adding .flat did not disable the border as expected.

https://bugzilla.gnome.org/show_bug.cgi?id=778905

gtk/theme/Adwaita/gtk-contained-dark.css

@@ -3789,7 +3789,7 @@ frame > border,
   padding: 0;
   border-radius: 0;
   border: 1px solid #1b1f20; }
-  frame > border.flat,
+  frame.flat > border,
   .frame.flat {
     border-style: none; }
   frame > border:backdrop,

gtk/theme/Adwaita/gtk-contained.css

@@ -3820,7 +3820,7 @@ frame > border,
   padding: 0;
   border-radius: 0;
   border: 1px solid #b6b6b3; }
-  frame > border.flat,
+  frame.flat > border,
   .frame.flat {
     border-style: none; }
   frame > border:backdrop,

gtk/theme/HighContrast/gtk-contained-inverse.css

@@ -2616,7 +2616,7 @@ frame border,
 .frame {
   border: 1px solid gray;
   padding: 0; }
-  frame border.flat,
+  frame.flat > border,
   .frame.flat {
     border-style: none; }
   frame border:backdrop,

gtk/theme/HighContrast/gtk-contained.css

@@ -2623,7 +2623,7 @@ frame border,
 .frame {
   border: 1px solid gray;
   padding: 0; }
-  frame border.flat,
+  frame.flat > border,
   .frame.flat {
     border-style: none; }
   frame border:backdrop,