After the first time you have launched Harmony, your preferences, along with numerous other options and switches in the user interface, are stored in a hidden directory on your computer.
Depending on your operating system, you can find your preference files in the following location:
Windows: C:\Users\[username]\AppData\Roaming\Toon Boom Animation\Toon Boom Harmony Essentials\full-1700-prefNOTE On Windows, the AppData folder is hidden. To display it in Explorer, in the top menu, open the View pane and check the Hidden Items checkbox.
macOS: /Users/[username]/Library/Preferences/Toon Boom Animation/Toon Boom Harmony Essentials/full-1700-prefNOTE On macOS, the Library folder is hidden. To display it in Finder, hold down the Alt key.