I sympathize with your problem - I've had to edit the heck out of the colors in my template too.
I've never been able to edit any of my CSS's from within Joomla. Just FTP the current CSS down to your local computer, edit it, then FTP the new version back up. Be sure to create a backup of the old version before you edit it.
If I may, here are some useful tips for editing the colors:
1) Install the Firefox web browser if you don't have it, then install the add-on called "Colorzilla". This will create an icon in the bottom left corner of Firefox. You can use it to hover over any part of your site and see the code for the color you're hovering over (for example #AAAAAA is white and #FFFFFF is black - or maybe the other way around, I forget). Then search for that color code in your CSS. I looked at your website. The color you want to change is #DDDDDD. However, looking at the rest of your site, I believe you may find that this color is being set in your Joomla template CSS, not your Fireboard CSS. That could be problematic, since changing the font color might affect your entire site, not just your Fireboard forums. You may want to consider changing the background color of Fireboard instead of the font color. Your background color is #F7F7F7.
2) Install the add-on called "Web Developer" into Firefox. This will create an icon in the bottom right corner of Firefox that will display the code for the current page. You can change the code in the window at the bottom and it will instantly show you what the web page would look like if you changed the code for real. So if you're not sure what effect changing the code will have, this is a safe way of finding out without actually changing anything.
Best of luck.
EDIT:
Well, since this was stickied, I'll add a little bit more info about installing Firefox add-ons. To install an add-on:
First, Firefox is downloadable from
www.mozilla.com.
1) From within Firefox, click Tools-->Add-Ons, then in the bottom right corner, click "Get Extensions".
2) In the "Search" box at the top, type the name of the add-on you're looking for. Or just browse them if you want - there are hundreds of cool add-ons available.
In order to make use of the Firefox add-ons you must, of course, be viewing your website with Firefox instead of another browser such as Internet Explorer, Opera, or Safari.