I am looking for a simple application that provides a countdown timer and follows the elementary OS HIGs.
I think you need a pomodoro App
Well there are some that are specifically made for elementary.
Tomato App is a time management application based on the Pomodoro technique, specially developed for the elementary OS 0.3 Freya and elementary OS 0.2 Luna systems. You can use the stable PPA now:
sudo apt-add-repository ppa:tomato-team/tomato-stable sudo apt-get update && sudo apt-get install tomato
Go For it
Yep I did go fast and recommended a basic app (tomato) but I have dug one more out with more features, but it is doesn't look as good compared to tomato.
It does have a more configurable timer. It is a simple and stylish productivity app, featuring a to-do list, merged with a timer that keeps your focus on the current task.
To install it you can use this ppa :
sudo add-apt-repository ppa:mank319/go-for-it && sudo apt-get update sudo apt-get install go-for-it
The github repo is at https://github.com/mank319/Go-For-It