Fix ag_window_set_theme()

It didn’t actually set the display theme in the AgWindow object

src/ag-window.c

@@ -1394,12 +1394,15 @@ ag_window_update_style_sheets(AgWindow *window)
 static void
 ag_window_set_theme(AgWindow *window, AgDisplayTheme *theme)
 {
-    gchar *css,
-          *css_final;
+    gchar           *css,
+                    *css_final;
+    AgWindowPrivate *priv = ag_window_get_instance_private(window);
 
     g_debug("Setting theme to %s", (theme) ? theme->name : "no theme");
     ag_window_clear_style_sheets(window);
 
+    priv->theme = theme;
+
     // Add the default style sheet
     ag_window_add_style_sheet(
             window,