Where are Additional CSS files stored

It’s stored in the database, within the wp_posts table, under the custom_css post type, where the post name is the theme slug. There you also have the related customize_changeset and revision post types.

The custom css post ID is also stored in the wp_options table under each theme mods, e.g. theme_mods_twentysixteen for Twenty Sixteen.

It’s informative to check out how the custom CSS fetched from the database with:

• wp_get_custom_css() that uses wp_get_custom_css_post()

and displayed through the wp_head action with:

• wp_custom_css_cb()