Source

File: wp-includes/theme.php

function wp_get_themes( $args = array() ) {
	global $wp_theme_directories;

	$defaults = array(
		'errors'  => false,
		'allowed' => null,
		'blog_id' => 0,
	);
	$args     = wp_parse_args( $args, $defaults );

	$theme_directories = search_theme_directories();

	if ( is_array( $wp_theme_directories ) && count( $wp_theme_directories ) > 1 ) {
		// Make sure the current theme wins out, in case search_theme_directories() picks the wrong
		// one in the case of a conflict. (Normally, last registered theme root wins.)
		$current_theme = get_stylesheet();
		if ( isset( $theme_directories[ $current_theme ] ) ) {
			$root_of_current_theme = get_raw_theme_root( $current_theme );
			if ( ! in_array( $root_of_current_theme, $wp_theme_directories, true ) ) {
				$root_of_current_theme = WP_CONTENT_DIR . $root_of_current_theme;
			}
			$theme_directories[ $current_theme ]['theme_root'] = $root_of_current_theme;
		}
	}

	if ( empty( $theme_directories ) ) {
		return array();
	}

	if ( is_multisite() && null !== $args['allowed'] ) {
		$allowed = $args['allowed'];
		if ( 'network' === $allowed ) {
			$theme_directories = array_intersect_key( $theme_directories, WP_Theme::get_allowed_on_network() );
		} elseif ( 'site' === $allowed ) {
			$theme_directories = array_intersect_key( $theme_directories, WP_Theme::get_allowed_on_site( $args['blog_id'] ) );
		} elseif ( $allowed ) {
			$theme_directories = array_intersect_key( $theme_directories, WP_Theme::get_allowed( $args['blog_id'] ) );
		} else {
			$theme_directories = array_diff_key( $theme_directories, WP_Theme::get_allowed( $args['blog_id'] ) );
		}
	}

	$themes         = array();
	static $_themes = array();

	foreach ( $theme_directories as $theme => $theme_root ) {
		if ( isset( $_themes[ $theme_root['theme_root'] . '/' . $theme ] ) ) {
			$themes[ $theme ] = $_themes[ $theme_root['theme_root'] . '/' . $theme ];
		} else {
			$themes[ $theme ] = new WP_Theme( $theme, $theme_root['theme_root'] );

			$_themes[ $theme_root['theme_root'] . '/' . $theme ] = $themes[ $theme ];
		}
	}

	if ( null !== $args['errors'] ) {
		foreach ( $themes as $theme => $wp_theme ) {
			if ( $wp_theme->errors() != $args['errors'] ) {
				unset( $themes[ $theme ] );
			}
		}
	}

	return $themes;
}