twenny is a very simple mac-menubar app created with rumps that reminds you to give your eyes a break every twenty minutes. It is based on the 202020-rule: Look at something that's 20 metres away for 20 seconds every 20 minutes to prevent digital eye strain.
Go to releases, download the latest twenny_vX.X.zip file, extract it, put it in your applications folder and you should be good to go! As seen in the video below, Apple sometimes blocks apps from unidentified developers from opening. Just follow the instructions in the video to install it anyways.
Here is the complete video tutorial: https://www.youtube.com/watch?v=jAxXJoHzDKU