This extension requires anvil.js version 0.9.* or greater.
Some bizarre issue in stylus means that it can't run within any directory ending in ".styl" which means I had to use a dash instead of a period. Sounds insane? It's true. Proofus: https://github.com/LearnBoost/stylus/issues/807
anvil install anvil-stylus
If this plugin is installed and enabled, it will automatically 'compile' files ending in .styl and produce .css files in your output folder.