'Customize' button in admin bar for CSS

Question

On the WordPress.org site I'm an admin for, we have a Customize button in the Admin bar (see very left):

This allows us to easily edit CSS on the front end without editing any of our files. I specifically remember reading that this feature allows us to write CSS that won't be overwritten by a theme update.

Now the time to update has come, and I'd like to absolutely confirm this. But I can't find any references to this button online.

What's this feature called? Is it on everyone's WordPress.org site, or is this some special feature of our theme or perhaps a plugin?

Answer 1

The WP Toolbar's Customize button will launch the Theme Customizer. This is standard on all WordPress sites, but it could be disabled via themes or plugins.

WordPress version 4.7 added the Custom CSS feature which will allow you to add your own styles to the activated theme. These styles will not be overridden when updating the theme.

...custom CSS safely stays available through updating and switching themes, starting fresh for each new theme.

Technically, it would be possible for a theme to stomp on any options or theme mods with an update, so you should always make a backup of your database and files before updating.