Ryan

Easily add custom JavaScript or CSS to a single post or page.
When activated, a new icon is added to the post edit page near the icon you use 
to add media to a post. Clicking the new icon brings up a screen where you can 
insert JavaScript and CSS that will be included with the current post.
Anything entered into the text field on the JavaScript tab will be inserted into 
the <head> section of your web page. Same is true of the CSS tab. This allows 
you to insert arbitrary JavaScript and CSS into any post or page you would 
like without having to resort to loading it on all pages.

Insert JavaScript and CSS

Adds fields to the post and page edit pages that allow you to insert custom JavaScript or CSS for that post or page.

salubrio

Intended users: Template Designers, Developer
If you wish to add any custom HTML to the bottom of each page’s head, then this plugin is for you. For example, you can add some custom CSS, a link to an external javascript file or something else.
The problem with editing the template directly is if you ever update the template, your changes will be lost. This plugin works around this issue.
<h3>Acknowledgements</h3>
Plugin Icon (CC BY 3.0) by <a href="https://www.iconfinder.com/denir" rel="nofollow ugc">DeniShop</a>

Add Code To Head

Add custom Javascript/HTML/CSS codes to the page head without editing the template.

Insert JavaScript and CSS vs Add Code To Head