Adds a custom css file to the form. To choose a css file, execute this piece and call the function ff_addCustomCSSFile('path/to/css/file') with the RELATIVE (not full!) path to your joomla installation.
Do not forget to call $this->execPieceByName('ff_InitLib') before!

Example:

global $mainframe;

$this->execPieceByName('ff_InitLib');
$this->execPieceByName('ff_addCustomCSSFile');

ff_addCustomCSSFile('templates/' . $mainframe->getTemplate() . '/css/template.css');