Stains and old dirt in the grouting can seldom be removed with domestic cleaners. If ugly tile stains are the only thing about your toilet you want to fix, just call in a professional cleaning company. They have industrial-strength cleaning solutions and the right tools for the job. For better hygiene, get a contractor to strip out and replace all the tile grouting. This route would be less complex and expensive than overlaying the tiles.
There are stylish, extra-thin tiles in the market that can give your toilet a fresh, clean look in less time and with less inconvenience than the usual renovation. The extra tile layer may raise the bathroom floor to almost the same level as the floor outside. You’d need to be extra careful when washing the toilet to prevent water from sloshing out the door.
You also have to be careful about the extra weight, i.e. the total weight or “loading” on the floor must not exceed a specified amount or the building’s structure might be stressed. Do inform your town council or the condo management, and get their approval before starting.