This package teaches outline-minor-mode to highlight section headings, without also highlighting top-level s-expressions.

To highlight only section headings forgo setting the built-in outline-minor-mode-highlight to append, or another non-nil value. Instead enable the equivalent feature provided by this package:

(use-package outline-minor-faces
  :after outline
  :config (add-hook 'outline-minor-mode-hook
                    #'outline-minor-faces-mode))

For non-lisp major modes the highlighting provided by this package and by the built-in support is essentiall the same, i.e., the first lines of top-level expressions are highlighted.

This package also defines separate faces for use in the minor mode. These faces are what gave this package its name, but nowadays they inherit from the built-in faces by default, and are preserved mostly for historic reasons, i.e., to avoid having to rename this package.

To further improve the appearance of collapsed sections, check out the backline package.