Source

File: wp-admin/includes/class-wp-theme-install-list-table.php

private function _get_theme_status( $theme ) {
		$status = 'install';

		$installed_theme = wp_get_theme( $theme->slug );
		if ( $installed_theme->exists() ) {
			if ( version_compare( $installed_theme->get( 'Version' ), $theme->version, '=' ) ) {
				$status = 'latest_installed';
			} elseif ( version_compare( $installed_theme->get( 'Version' ), $theme->version, '>' ) ) {
				$status = 'newer_installed';
			} else {
				$status = 'update_available';
			}
		}

		return $status;
	}