Unfortunately, there isn't one yet.
I've asked myself the same thing occasionally but I've found a handful of examples through the years that I've drawn some inspiration and learned through. Unfortunately, most authors aren't putting their all or some of their work public. Also, there's been a couple small changes in carto, some of the code may not be 100% up to date or may not take advantage of carto's current capabilities and you'll also have to change things as people use different data types (postgis queries, shapefiles, json, etc) for your needs.
nice Transit map by Code for america participant jlord - https://github.com/codeforamerica/Transit-Map-in-TileMill
https://github.com/mapbox/osm-bright - A great way to start learning carto. This is what really helped me to learn carto and I often use it as a base layer for projects or begin with it for a project.
A fork of it.
Pandonia - style made by flickr
Some examples from Dane Springmeyer, Tilemill and mapnik developer These are very experimental and push the bounds of what can be done in Carto.
A mapping style for humanitarian contexts from HOT, the humanitarian openstreetmap team.
Additionally, the cartocss port of the OSM default style mentioned by maning is a nice work.
Lastly, this one in my github repo is a decent take off of osm-bright as well:
Experimenting with these styles and modifying them will really help you learn.