Recently I needed to change my WeBrick’s application root from /public to something else.  If you need to have it running under a different directory, all of your routing to public stylesheets, images, etc. will have errors.

In your Rails directory open the file /vendor/rails/railties/lib/commands/servers/webrick.rb (or where you have installed Rails):

...
OPTIONS = {
  :port         => 3000,
  :ip           => "0.0.0.0",
  :environment  => (ENV['RAILS_ENV'] || "development").dup,
  :server_root  => File.expand_path(RAILS_ROOT + "/public_html/"),
  :server_type  => WEBrick::SimpleServer,
  :charset      => "UTF-8",
  :mime_types   => WEBrick::HTTPUtils::DefaultMimeTypes,
  :debugger     => false
}
...

You may change the settings for WeBrick including the port, environment and server root to use. Very handy stuff!