I don't think it is a good idea to try to 'auto-load' a css. The css
is used in a layout, but it's up to the user to specify which layout
or layouts she wants to use your plugin in. Also, your css rules may
colide with someone else's code and if you auto-load it, it would be
very hard to catch and debug.
I would instead use your plugin's install.rb to move the css to public/
stylesheets, and include instructions on how to include the CSS in