So in the Forum Wrapper put:
<div id=whatever-div-id-you-want>FORUM DESCRIPTION HERE</div>
The id can contain letters, _ and - but no spaces.
Save that and go to the colours and styles part, click on the style sheet and at the bottom paste:
#whatever-div-id-you-want {
color: @banner_text_color;
text-shadow: @banner_text_shadow;
text-decoration: @banner_text_decoration;
font: @banner_text_font;
font-variant: @banner_text_caps;
text-transform: @banner_text_case;
font-size: WHATEVER FONT SIZE YOU WANT;
}
I copied the variables from the banner section so that the description would match the look of the Forum Title, you can remove any of these you want and choose to put absolute values like georgia or whatever. It's also possible to add variable like alignment if you want them.