emacs/mmm-modeのインストール - kobapan @ wiki

$ sudo apt-get install mmm-mode

.emacsに追記

;; ---------------------------------------------------------------------------
;; MMM mode
;; ---------------------------------------------------------------------------
(require 'mmm-auto)
(setq mmm-global-mode 'maybe)
(set-face-background 'mmm-default-submode-face nil) ;背景色は不要
(mmm-add-classes
 '((embedded-css
    :submode css-mode
    :front "<style[^>]*>"
    :back "</style>")))
(mmm-add-mode-ext-class nil "\\.html\\'" 'embedded-css)