Are you using a customized skin? If not, check for the existance of a styleforum.css file. If it is there, use it instead of forummodule.css. A few releases ago, Joe A. changed all of the skins to use one css file for the mojoPortal core and then renamed all the feature specific css files to style[feature].css

Also, make sure you have following set in your user.config while you are adjusting your skin:

<add key="CombineCSS" value="false" />
<add key="MinifyCSS" value="false" />
<add key="CacheCssOnServer" value="false" />
<add key="CacheCssInBrowser" value="false" />

Hope this helps,
Joe D.