In order to have a Font available in AEM 6.3 Forms, you need to do two steps:
1. Make the font known to AEM by providing the path to it: go to http://[server]:[port_number]/system/console/configMgr and edit "CQ-DAM-Handler-Gibson Font Manager Service". Add the path to your font library (e.g. "/Library/Fonts" under macOS). Alternatively, you can add the font to "[AEM Install Dir]/crx-quickstart/fonts".
2. Next, edit "Adaptive Form Theme Configuration Service". Add a new entry for each font you want to use. Make sure the Spelling of the official font name is correct, replacing spaces with "\ " (backslash and space).
3. Restart your server and now you should be able to select the font in the Theme or Form editor.