Add GTK3 version of our custom styles

Although GTK3 still have gtk_rc_parse_string(), it doesn't work anymore
for our overrides, so register proper CSS for them.

Makefile.am

@@ -24,8 +24,13 @@ SYS_DATA_FILES = \
 	$(srcdir)/data/templates/* \
 	$(srcdir)/data/templates/files/* \
 	$(srcdir)/data/colorschemes/* \
-	$(top_srcdir)/data/geany.glade \
+	$(top_srcdir)/data/geany.glade
+
+if GTK3
+	$(top_srcdir)/data/geany.css
+else
 	$(top_srcdir)/data/geany.gtkrc
+endif
 
 EXTRA_DIST = \
 	autogen.sh \

configure.ac

@@ -67,6 +67,7 @@ AS_IF([test "x$enable_gtk3" = xyes],
 	   gtk_min_version=3.0],
 	  [gtk_package=gtk+-2.0
 	   gtk_min_version=2.16])
+AM_CONDITIONAL([GTK3], [test "x$gtk_package" = "xgtk+-3.0"])
 
 # GTK/GLib/GIO checks
 gtk_modules="$gtk_package >= $gtk_min_version glib-2.0 >= 2.20"

data/geany.css

@@ -0,0 +1,25 @@
+/* custom GTK3 CSS for Geany */
+
+/* make close button on the editor's tabs smaller */
+#geany-close-tab-button {
+	-GtkWidget-focus-padding: 0;
+	-GtkWidget-focus-line-width: 0;
+	-GtkButton-default-border: 0;
+	-GtkButton-default-outside-border: 0;
+	-GtkButton-inner-border: 0;
+	padding: 0;
+}
+
+/* use monospaced font in search entries for easier reading of regexp (#1907117) */
+#GeanyDialogSearch GtkEntry {
+	font-family: monospace;
+}
+
+/* set red background for GtkEntries showing unmatched searches */
+#geany-search-entry-no-match {
+	color: #fff;
+	background: #ff6666;
+}
+#geany-search-entry-no-match:selected {
+	background-color: #771111;
+}

src/ui_utils.c

@@ -2260,10 +2260,30 @@ void ui_init_builder(void)
 
 static void init_custom_style(void)
 {
+#if GTK_CHECK_VERSION(3, 0, 0)
+	gchar *css_file = g_build_filename(app->datadir, "geany.css", NULL);
+	GtkCssProvider *css = gtk_css_provider_new();
+	GError *error = NULL;
+
+	if (! gtk_css_provider_load_from_path(css, css_file, &error))
+	{
+		g_warning("Failed to load custom CSS: %s", error->message);
+		g_error_free(error);
+	}
+	else
+	{
+		gtk_style_context_add_provider_for_screen(gdk_screen_get_default(),
+			GTK_STYLE_PROVIDER(css), GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
+	}
+
+	g_object_unref(css);
+	g_free(css_file);
+#else
 	gchar *gtkrc_file = g_build_filename(app->datadir, "geany.gtkrc", NULL);
 
 	gtk_rc_parse(gtkrc_file);
 	g_free(gtkrc_file);
+#endif
 }