So you're running a Rails application and want to spruce up your YARD documentation. Here's the guide I wish I had.
Add yard-activerecord and yard-activesupport-concern to your Gemfile, add --plugin activerecord and --plugin activesupport-concern to your .yardopts flags, and db/schema.rb to the end of your .yardopts sources. Your models' attributes and assocations should now be included in the documentation.
To modify your YARD template, create doc-src/templates/default/fulldoc/html, then add --template-path doc-src/templates to your project's .yardopts. You can now create a doc-src/templates/default/fulldoc/html/css/common.css and the styles will be included (but overwrites need to be !important).