How to highlight a custom command in emacs with auctex

latex-custom-match-command.el

;; Reference: https://www.flannaghan.com/2013/01/09/command-highlight

;;This option will hightlight the content inside the parens

(setq font-latex-user-keyword-classes
          '(("my-warning-commands"
                (("fixme" "{"))
                (:background "red" :foreground "black")
                command)))


;; This option will hightlight the command itself

(setq font-latex-match-warning-keywords
     '(("fixme" "{") ))