diff --git a/maturita.h b/maturita.h
index 0ff3136..604bc51 100644
--- a/maturita.h
+++ b/maturita.h
@@ -7,6 +7,11 @@
 #include <pango/pangocairo.h>
 #include <math.h>
 
+/* GLib on Ubuntu 22.04 predates G_APPLICATION_DEFAULT_FLAGS. */
+#ifndef G_APPLICATION_DEFAULT_FLAGS
+#define G_APPLICATION_DEFAULT_FLAGS G_APPLICATION_FLAGS_NONE
+#endif
+
 #define NUM_UNITS     10
 #define NUM_UNLOCKED   3   /* first NUM_UNLOCKED units (index 0..2) are open */
 #define MAX_UNIT_EX   20   /* largest exercise count any unit may have       */
diff --git a/theme.c b/theme.c
index 57f36c7..db9e3d3 100644
--- a/theme.c
+++ b/theme.c
@@ -394,7 +394,12 @@ void apply_theme(void) {
             GTK_STYLE_PROVIDER_PRIORITY_USER);
     }
 
+    #if GTK_CHECK_VERSION(4, 12, 0)
     gtk_css_provider_load_from_string(theme_provider, css);
+#else
+    /* GTK 4.6 (Ubuntu 22.04) — load_from_string arrived in 4.12. */
+    gtk_css_provider_load_from_data(theme_provider, css, -1);
+#endif
     g_free(css);
 
     refresh_welcome_heading();