wp-includes/customize/class-wp-customize-custom-css-setting.php:185Store the CSS setting value in the custom_css custom post type for the stylesheet.
$valuestringint|false public function update( $value ) { // Restores the more descriptive, specific name for use within this method. $css = $value; if ( empty( $css ) ) { $css = ''; } $r = wp_update_custom_css_post( $css, array( 'stylesheet' => $this->stylesheet, ) ); if ( $r instanceof WP_Error ) { return false; } $post_id = $r->ID; // Cache post ID in theme mod for performance to avoid additional DB query. if ( $this->manager->get_stylesheet() === $this->stylesheet ) { set_theme_mod( 'custom_css_post_id', $post_id ); } return $post_id; }Introduced in 4.7.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/customize/class-wp-customize-custom-css-setting.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.