The auto-dim-other-buffers-mode is a global minor mode which makes non-current buffer less prominent making it more apparent which window has a focus.

The preferred way to install the mode is by installing a package form MELPA:

M-x package-install RET auto-dim-other-buffers RET

Once installed, the mode can be turned on (globally) with:

M-x auto-dim-other-buffers-mode RET

To make the mode enabled every time Emacs starts, add the following to Emacs initialisation file (~/.emacs or ~/.emacs.d/init.el):

(add-hook 'after-init-hook (lambda ()
  (when (fboundp 'auto-dim-other-buffers-mode)
    (auto-dim-other-buffers-mode t))))

To configure how dimmed buffers look like, customise auto-dim-other-buffers-face. This can be accomplished by:

M-x customize-face RET auto-dim-other-buffers-face RET

The auto-dim-other-buffers-mode is a global minor mode which makes non-current buffer less prominent making it more apparent which window has a focus.