Convert global CSS to inline CSS.

http://www.contao-community.de/showthread.phpß12811-Avisota-Umfangreiches-Newslettersystem-Version-0.4-beta1-First-Production-Release&p=130398&viewfull=1#post130398

http://blog.verkoyen.eu/blog/p/detail/convert-css-to-inline-styles-with-php

See http://symfony.com/doc/current/components/css_selector.html