Pomodorino is a lightweight, simple Pomodoro timer system tray application written using Python 3 and GTK 3.
What sets it apart is that it’s totally FOSS (licensed under GPLv3+) and really lightweight. Alternative Linux apps that I could find were either paid and proprietrary or they used rather heavy technologies like Electron.
Using Pomodorino should be rather straight-forward if you know about the Pomodoro Technique® (which is a registered trademark of Francesco Cirillo).
For more information, see the README, the manual, and the wiki.