The folder /Library/ColorSync/Profiles does, or should exist on macOS 10.15.
(Note the / at the beginning, signifying the root level of the volume, the US spelling of 'color', and the correct capitalization.)
It's possible, or even likely, that the similar path inside the User account may not be there. E.g. /Users/<yourUserName>/Library/ColorSync/...
As of Xcode Version 11.6 (11E708), the file extension for color themes is xccolortheme.
The path to the included themes is:
The path to a user's custom themes is the same as in the accepted answer: