diff -r b679cc6059ab -r d96ba820083a ui/gtk/toolkit.c
--- a/ui/gtk/toolkit.c	Sun Sep 29 15:01:14 2024 +0200
+++ b/ui/gtk/toolkit.c	Sun Sep 29 15:55:56 2024 +0200
@@ -394,7 +394,12 @@
         size_t numClasses = cx_strsplit_a(cxDefaultAllocator, cx_str(style_classes), CX_STR(" "), 128, &cls);
         for(int i=0;i<numClasses;i++) {
             cxmutstr m = cx_strdup(cls[i]);
+#if GTK_MAJOR_VERSION >= 4
             gtk_widget_add_css_class(widget, m.ptr);
+#elif GTK_MAJOR_VERSION >= 3
+            GtkStyleContext *ctx = gtk_widget_get_style_context(widget);
+            gtk_style_context_add_class(ctx, m.ptr);
+#endif
             free(m.ptr);
         }
         free(cls);