Source

File: wp-admin/includes/class-wp-ms-themes-list-table.php

public function _search_callback( $theme ) {
		static $term = null;
		if ( is_null( $term ) ) {
			$term = wp_unslash( $_REQUEST['s'] );
		}

		foreach ( array( 'Name', 'Description', 'Author', 'Author', 'AuthorURI' ) as $field ) {
			// Don't mark up; Do translate.
			if ( false !== stripos( $theme->display( $field, false, true ), $term ) ) {
				return true;
			}
		}

		if ( false !== stripos( $theme->get_stylesheet(), $term ) ) {
			return true;
		}

		if ( false !== stripos( $theme->get_template(), $term ) ) {
			return true;
		}

		return false;
	}