Manage and live-preview theme-specific and plugin-specific Custom CSS in the Customizer.
Theme CSS is stored as a
theme_mod, meaning it is a theme-specific option, part of the theme_mod_$theme option in the database. Each theme has its own
theme_mod for the custom CSS, so if you switch to a new theme, the theme-specific custom CSS will be empty. When you switch back to a previously customized theme, the CSS that you added to it will still be there.
Plugin CSS is stored as a regular
option in the database. It is used for every theme, so it's best used for things that are plugin-related or anything else you want to persist between different themes. Most Custom CSS plugins store the CSS as an option, but that means that theme-specific CSS must be deleted and lost when you switch themes.
The Customizer features many improvements in WordPress 4.0, including the textarea control type that this plugin uses.