diff --git a/README.org b/README.org
index cc3d053..0ec825f 100644
--- a/README.org
+++ b/README.org
@@ -1276,10 +1276,36 @@ the next source code block shows.
       (set-face-attribute 'default nil :height height)))
 #+end_src
 
-This setup prefers the ~leuven~ and ~leuven-dark~ themes from [[https://melpa.org/#/][MELPA]], because the
-very popular ~modus-operandi~ and ~modus-vivendi~ themes feel quirky: for
-instance those themes fail to display ~hl-line-mode~ properly with Emacs-27.2 on
-Darwin.
+Allow swapping fhe foreground and background colors of the default face on all
+frames.
+
+#+begin_src emacs-lisp
+  (defun my-invert-default-face ()
+    "Invert the default face."
+    (interactive)
+    (invert-face 'default))
+#+end_src
+
+Enable =rainbow-mode= to colorize color names in buffers for debugging.
+
+#+begin_src emacs-lisp
+  (when (fboundp 'rainbow-mode)
+    (custom-set-variables
+     '(rainbow-x-colors-major-mode-list
+       '(c++-mode
+         c-mode
+         emacs-lisp-mode
+         inferior-emacs-lisp-mode
+         java-mode
+         lisp-interaction-mode
+         org-mode
+         python-mode)))
+    (rainbow-mode +1))
+#+end_src
+
+This setup prefers the ~leuven~ and ~leuven-dark~ themes because the
+~modus-operandi~ and ~modus-vivendi~ themes feel quirky: for instance those
+themes fail to display ~hl-line-mode~ properly with Emacs-27.2 on Darwin.
 
 [[https://emacs.stackexchange.com/questions/17431/how-do-i-change-portions-of-a-custom-theme][How to change custom theme faces]]