Add wxWindow::ApplyCssStyle() helper to wxGTK

This is useful for styling GTK+ 3 widgets.

See #17089.

include/wx/gtk/window.h

@@ -14,6 +14,7 @@
 #ifdef __WXGTK3__
     typedef struct _cairo cairo_t;
     typedef struct _GtkStyleProvider GtkStyleProvider;
+    typedef struct _GtkCssProvider GtkCssProvider;
     #define WXUNUSED_IN_GTK2(x) x
     #define WXUNUSED_IN_GTK3(x)
 #else
@@ -409,7 +410,11 @@ protected:
     // Copies m_children tab order to GTK focus chain:
     void RealizeTabOrder();
 
-#ifndef __WXGTK3__
+#ifdef __WXGTK3__
+    // Use the given CSS string for styling the widget. The provider must be
+    // allocated, and remains owned, by the caller.
+    void ApplyCssStyle(GtkCssProvider* provider, const char* style);
+#else // GTK+ < 3
     // Called by ApplyWidgetStyle (which is called by SetFont() and
     // SetXXXColour etc to apply style changed to native widgets) to create
     // modified GTK style with non-standard attributes.

src/gtk/window.cpp

@@ -4426,7 +4426,21 @@ PangoContext *wxWindowGTK::GTKGetPangoDefaultContext()
     return gtk_widget_get_pango_context( m_widget );
 }
 
-#ifndef __WXGTK3__
+#ifdef __WXGTK3__
+void wxWindowGTK::ApplyCssStyle(GtkCssProvider* provider, const char* style)
+{
+    wxCHECK_RET(m_widget, "invalid window");
+
+    gtk_style_context_remove_provider(gtk_widget_get_style_context(m_widget),
+                                      GTK_STYLE_PROVIDER(provider));
+
+    gtk_css_provider_load_from_data(provider, style, -1, NULL);
+
+    gtk_style_context_add_provider(gtk_widget_get_style_context(m_widget),
+                                   GTK_STYLE_PROVIDER(provider),
+                                   GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
+}
+#else // GTK+ < 3
 GtkRcStyle* wxWindowGTK::GTKCreateWidgetStyle()
 {
     GtkRcStyle *style = gtk_rc_style_new();