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, Admins
If you wish to add any custom HTML to each page’s header, then this plugin is for you.
This is useful for verifying you are the owner of the website to services such as Mailchimp or Google. You can quickly add the verification codes to your page header without having to edit your site’s template.
In general, you can add custom CSS, a link to an external JavaScript file or something else. While it is generally recommended to create a child template if you’re going to make extensive, permanent changes to a template, there may be instances where a small change or two is needed that wouldn’t justify the creation of a child template—or your current template might not support child templates. You should nearly always avoid editing a template directly, because your changes will be lost when you next update the template.
This plugin is not affected by template changes.
<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