Sorry, I know I’m missing something basic here, but haven’t been able to figure it out yet.
Whatever I change isn’t having an effect at all. I’ve edited the default.user.less file in /Applications/TaskPaper3.app/Contents/Resources. I’ve changed the name of the file to user.less, even to my .less.
On this system, from TP3 163, choosing File > Open Application Folder from the main menu takes me to the user.less in a different directory to the one you describe.