Remove hardcoded minibuffer-prompt face.

counsel.el: (counsel-default-outline): new face.
(counsel-outline--add-face): use counsel-default-outline.

counsel.el

@@ -4256,7 +4256,7 @@ the face to apply."
                 (custom (nth (1- level)
                              (or custom-faces counsel-outline-custom-faces)))
                 (org (format "counsel-org-outline-%d" level))
-                (t 'minibuffer-prompt))))
+                (t 'counsel-default-outline))))
     (when face
       (put-text-property 0 (length name) 'face face name)))
   name)
@@ -4719,6 +4719,10 @@ as arguments."
   '((t :inherit org-level-8))
   "Face for displaying org headings.")
 
+(defface counsel-default-outline
+  '((t :inherit minibuffer-prompt))
+  "Face for displaying default headings.")
+
 (defvar counsel-linux-apps-faulty nil
   "List of faulty desktop files.")