windowPreview: Overhaul chrome style

- increase close button size
 - use custom close icon that's better suited
   for the non-default 24pxp size
 - use translucent light gray for both close button
   and window caption

https://gitlab.gnome.org/Teams/Design/os-mockups/-/issues/81

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1605>

js/ui/windowPreview.js

@@ -354,7 +354,7 @@ var WindowPreview = GObject.registerClass({
         this._closeButton = new St.Button({
             visible: false,
             style_class: 'window-close',
-            child: new St.Icon({ icon_name: 'window-close-symbolic' }),
+            child: new St.Icon({ icon_name: 'window-close-24-symbolic' }),
         });
         this._closeButton.add_constraint(new Clutter.BindConstraint({
             source: this._windowContainer,