Modifying theme to add custom header

hi, i’d like to use as theme, but i need some special .css included in all posts/entry page etc., so i found this but somehow, it is not working.

The change is pretty simple. I wanted to put some custom JavaScript in the of a site I’m building, but I didn’t want to have to edit the theme or make a copy of layouts/_default/baseof.html , since later changes to the theme may break it. This change allows code to be insterted into by creating a layouts/partial/head-custom.html .

i created a layouts/partial/head-custom.html, but it is not included. I echoed “TEST” in it, but its not included. What am i doing wrong?

partial or partials?

thanks, in partials its working, so its a documentation error