The style property deprecated.

...
(mousepad:7559): Gtk-WARNING **: Theme parsing error: <data>:2:29: The style property GtkButton:default-border is deprecated and shouldn't be used anymore. It will be removed in a future version

(mousepad:7559): Gtk-WARNING **: Theme parsing error: <data>:3:37: The style property GtkButton:default-outside-border is deprecated and shouldn't be used anymore. It will be removed in a future version

(mousepad:7559): Gtk-WARNING **: Theme parsing error: <data>:4:27: The style property GtkButton:inner-border is deprecated and shouldn't be used anymore. It will be removed in a future version

(mousepad:7559): Gtk-WARNING **: Theme parsing error: <data>:5:31: The style property GtkWidget:focus-line-width is deprecated and shouldn't be used anymore. It will be removed in a future version

(mousepad:7559): Gtk-WARNING **: Theme parsing error: <data>:6:28: The style property GtkWidget:focus-padding is deprecated and shouldn't be used anymore. It will be removed in a future version
...

As seen with GTK+ 3.20.9 and 3.22.4.

---
 mousepad/mousepad-close-button.c | 24 ++----------------------
 1 file changed, 2 insertions(+), 22 deletions(-)

diff --git a/mousepad/mousepad-close-button.c b/mousepad/mousepad-close-button.c
index 6405fae..f7881c1 100644
--- a/mousepad/mousepad-close-button.c
+++ b/mousepad/mousepad-close-button.c
@@ -40,20 +40,7 @@ mousepad_close_button_style_set (GtkWidget *widget,
 static void
 mousepad_close_button_class_init (MousepadCloseButtonClass *klass)
 {
-#if GTK_CHECK_VERSION(3, 0, 0)
-  static const gchar *button_style =
-    "* {\n"
-    "  -GtkButton-default-border: 0;\n"
-    "  -GtkButton-default-outside-border: 0;\n"
-    "  -GtkButton-inner-border: 0;\n"
-    "  -GtkWidget-focus-line-width: 0;\n"
-    "  -GtkWidget-focus-padding: 0;\n"
-    "  padding: 0;\n"
-    "}\n";
-
-  klass->css_provider = gtk_css_provider_new ();
-  gtk_css_provider_load_from_data (klass->css_provider, button_style, -1, NULL);
-#else
+#if ! GTK_CHECK_VERSION(3, 20, 0)
   GtkWidgetClass *widget_class;
 
   gtk_rc_parse_string (
@@ -77,14 +64,7 @@ mousepad_close_button_init (MousepadCloseButton *button)
 {
   GtkWidget *image;
 
-#if GTK_CHECK_VERSION(3, 0, 0)
-  GtkStyleContext *context;
-
-  context = gtk_widget_get_style_context (GTK_WIDGET (button));
-  gtk_style_context_add_provider (context,
-                                  GTK_STYLE_PROVIDER (MOUSEPAD_CLOSE_BUTTON_GET_CLASS(button)->css_provider),
-                                  GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
-#else
+#if ! GTK_CHECK_VERSION(3, 20, 0)
   gtk_widget_set_name (GTK_WIDGET (button), "mousepad-close-button");
 #endif
 
-- 
2.10.2