Bluehost re-installed your website instead of just removing the functions.php file?
Adding CSS can’t crash your website, but PHP can. So you’re safe making CSS changes.
If you’re trying to use a sticky navigation, you can start with one of our child themes (Mantle, Exhibit) and then customize from there.
Another option is using a plugin like this: https://wordpress.org/plugins/sticky-menu-or-anything-on-scroll/