How do I create a custom theme for MacOS app? Templates? Examples?

- Seeing in the settings of the MacOS app that custom themes can be uploaded - After some Googling, however, I can't find any documentation on this - I understanding how to modify the web app using custom CSS via a Chrome Extension but really awesome that the MacOS app can be modified too ... I just don't know how! - I tried to look in my Mac library ApplicationSupport folder for something like .com.TickTick and .com.Dida thinking that is a possible place but no luck there!