Got it!
I have been struggling with this. Fireboard will carry over the default text color in your template's .css file. If the default color is white, or very light, it doesn't really work.
However, you can override it in the forum.css file in the fireboard template directory - i.e. /components/com_fireboard/template/default/
I added the the line "#fireboard {color: #000000;}" in the code below which changes the default text color to black...(or whatever you want).
/**
* @version $Id: forum.css 499 2007-12-16 22:33:57Z fxstein $
* Fireboard Component
* @package Fireboard
* @Copyright (C) 2006 - 2007 Best Of Joomla All rights reserved
* @license
www.gnu.org/copyleft/gpl.html GNU/GPL
* @link
www.bestofjoomla.com
**/
#fireboard {color: #000000;}
#fireboard a { text-decoration:none; font-weight:normal;color:#404040;}
#fireboard a:link , #fireboard a:visited{ color:#404040;}
#fireboard a:hover { color: #87bc01;}
#fireboard .fbxs {font-size: 9px;}
#fireboard .fbs {font-size: 10px;}
#fireboard .fbm {font-size: 11px;}
#fireboard .fbl {font-size: 12px;}
#fireboard .fbxl {font-size: 13px;}
#fireboard .fbxxl {font-size: 14px;}