leemeichin

;; Sets the default mode-line. Remove '-default' if you want to test your changes on-the-fly with (eval-buffer)
(setq-default mode-line-format
      '(
        ;; add a noticeable red block that says 'READ ONLY' when the file's, er, read only
        (:eval
         (when (eql buffer-read-only t)
             ;; propertize adds metadata to text, so you can add colours and formatting, amongst other things
             (propertize " READ ONLY " 'face
                         '(:background "color-88" :foreground "white" :weight bold))))
        ;; show the buffer filename, with a green background when unmodified/saved and a red one when modified