highlight-focus

Source: http://amitp.blogspot.com/2013/05/emacs-highlight-active-buffer.html

This is an Emacs Lisp package for highlighting the active buffer by dynamically changing the value of a face property (defaults to the :background property of the default face).

Usage

;; if you use el-get
(el-get-bundle kriyative/highlight-focus)

;; if you use use-package
(use-package highlight-focus
  :config
  ;; set the background of the mode-line
  (setq highlight-focus:face 'mode-line
        highlight-focus:face-property :background
        highlight-focus:face-property-value "darkgreen"))