Just use a face for customizing the unfocused region

Use font-lock-comment-face as a default. Maybe this fixes #21?

focus.el

@@ -38,17 +38,6 @@
   :group 'font-lock
   :prefix "focus-")
 
-(defcustom focus-dimness 0
-  "Amount of dimness in out of focus sections is determined by this integer.
-
-A positive value increases the dimness of the sections.
-A negative value decreases the dimness.
-
-The default is 0 which means a 50/50 mixture of the background
-and foreground color."
-  :type '(integer)
-  :group 'focus)
-
 (defcustom focus-mode-to-thing '((prog-mode . defun) (text-mode . sentence))
   "An associated list between mode and thing.
 
@@ -73,7 +62,7 @@ Things that are defined include `symbol', `list', `sexp',
 (defvar focus-cursor-type cursor-type
   "Used to restore the users `cursor-type'")
 
-(defvar focus-dim-color nil
+(defvar focus-unfocused-face font-lock-comment-face
   "Overrides the color used for dimmed text.")
 
 (defvar-local focus-current-thing nil
@@ -105,28 +94,6 @@ The timer calls `focus-read-only-hide-cursor' after
   "Return the current bounds, based on `focus-get-thing'."
   (bounds-of-thing-at-point (focus-get-thing)))
 
-(defun focus-average-colors (color &rest colors)
-  "Takes an average of the colors given by argument.
-Argument COLOR is a color name, and so are the COLORS; COLOR is
-there to ensure that the the function receives at least one
-argument."
-  (let* ((colors (cons color colors))
-         (colors (mapcar 'color-name-to-rgb colors))
-         (len    (length colors))
-         (sums   (apply 'cl-mapcar '+ colors))
-         (avg    (mapcar (lambda (v) (/ v len)) sums)))
-    (apply 'color-rgb-to-hex avg)))
-
-(defun focus-make-dim-color ()
-  "Return a dimmed color relative to the current theme."
-  (let ((background (face-attribute 'default :background))
-        (foreground (face-attribute 'default :foreground))
-        (backgrounds (if (> focus-dimness 0)    focus-dimness  1))
-        (foregrounds (if (< focus-dimness 0) (- focus-dimness) 1)))
-    (apply 'focus-average-colors
-           (append (make-list backgrounds background)
-                   (make-list foregrounds foreground)))))
-
 (defun focus-move-focus ()
   "Move the focused section according to `focus-bounds'.
 
@@ -152,9 +119,8 @@ adds `focus-move-focus' to `post-command-hook'."
     (setq focus-pre-overlay  (make-overlay (point-min) (point-min))
           focus-post-overlay (make-overlay (point-max) (point-max))
           focus-buffer (current-buffer))
-    (let ((color (or focus-dim-color (focus-make-dim-color))))
+    (mapc (lambda (o) (overlay-put o 'face focus-unfocused-face))
-      (mapc (lambda (o) (overlay-put o 'face (cons 'foreground-color color)))
+          (list focus-pre-overlay focus-post-overlay))
-            (list focus-pre-overlay focus-post-overlay)))
     (add-hook 'post-command-hook 'focus-move-focus nil t)
     (add-hook 'change-major-mode-hook 'focus-terminate nil t)))
 
@@ -276,11 +242,6 @@ It cleans up the `focus-read-only-blink-timer' and hooks."
   :keymap (let ((map (make-sparse-keymap)))
             (define-key map (kbd "C-c C-q") 'focus-read-only-mode)
             map)
-  (unless (or (and (color-defined-p (face-attribute 'default :background))
-                   (color-defined-p (face-attribute 'default :foreground)))
-              focus-dim-color)
-    (message "Can't enable focus mode when no theme is loaded. Try setting focus-dim-color!")
-    (setq focus-mode nil))
   (if focus-mode (focus-init) (focus-terminate)))
 
 ;;;###autoload