Things are trickier when you count down because you need to know when to stop! (Or, in this case, when to change styles.)
1. Add a sound when countdown reaches 0.
2. Prompt user for initial number of seconds (possibly as hours and minutes and seconds).
3. Style it as you wish, obvs.
4. Stop the timer when it reaches 0.
5. Play the song, "Final Countdown." HAHAHAHAHAHAH!
6. Preset buttons, like for "lunch break", that start the countdown for a set number of seconds (maybe 1 hour for lunch).
7. Display the time when the countdown will reach 0.