Adding Custom CSS without touching Masterpage

You can do that using the Delegate-Control mechanism and the builtIn Control "CSSRegistration".

Simply create a Feature (Web- or Sitescoped) and a Module and put the following Control Definition into it:

  
    URLToCSSFile
  

If you start your url without "/", it will assume your CSS-File lies within the "Styles"-Folder in _layouts and prefixes the corresponding parts automatically