Source

File: wp-includes/theme.php

function get_editor_stylesheets() {
	$stylesheets = array();
	// Load editor_style.css if the current theme supports it.
	if ( ! empty( $GLOBALS['editor_styles'] ) && is_array( $GLOBALS['editor_styles'] ) ) {
		$editor_styles = $GLOBALS['editor_styles'];

		$editor_styles = array_unique( array_filter( $editor_styles ) );
		$style_uri     = get_stylesheet_directory_uri();
		$style_dir     = get_stylesheet_directory();

		// Support externally referenced styles (like, say, fonts).
		foreach ( $editor_styles as $key => $file ) {
			if ( preg_match( '~^(https?:)?//~', $file ) ) {
				$stylesheets[] = esc_url_raw( $file );
				unset( $editor_styles[ $key ] );
			}
		}

		// Look in a parent theme first, that way child theme CSS overrides.
		if ( is_child_theme() ) {
			$template_uri = get_template_directory_uri();
			$template_dir = get_template_directory();

			foreach ( $editor_styles as $key => $file ) {
				if ( $file && file_exists( "$template_dir/$file" ) ) {
					$stylesheets[] = "$template_uri/$file";
				}
			}
		}

		foreach ( $editor_styles as $file ) {
			if ( $file && file_exists( "$style_dir/$file" ) ) {
				$stylesheets[] = "$style_uri/$file";
			}
		}
	}

	/**
	 * Filters the array of URLs of stylesheets applied to the editor.
	 *
	 * @since 4.3.0
	 *
	 * @param string[] $stylesheets Array of URLs of stylesheets to be applied to the editor.
	 */
	return apply_filters( 'editor_stylesheets', $stylesheets );
}