The Appearance/Customise/Additional CSS option places the script inline in your main HTML file, after compressing and optionally compiling the LESS or SASS.
However, for a standard theme the
wp_enqueue_style functions just include the raw
css file via the bog-standard
link mechanism. Googling shows me a number of plugins that will merge and minify multiple
css files, but WordPress already has that functionality built-in, so is there any way to take advantage of it?
Read more here: Inlining stylesheets like "Additional CSS" does