All CSS changes are made in the pattern library. The build process for the docs then pulls the compiled CSS from the pattern library into the docs build.
cd perch-docs/patterns
npm install
and then to run it:
gulp
this should watch for changes and rebuild the CSS.
cd perch-docs
npm install
Once the pattern library is running (and therefore fresh CSS is being built) you can build the docs:
node build
The built output goes into the public folder. You can point a web server at that. The node module http-server is quite useful for this.