:rtfm: It's right there in the forum plugin's installation instructions:
Step 7 - Update your themes css - style.css file
* The forum is using the plugin CSS that is distributed with the GL 1.4.1 professional theme. If you are running a GL 1.4.1 site and using the stock professional theme or a 1.4.1 certified theme, you will only need to add a few new CSS styles declarations for the codeblock and quote formatting feature.
* If you have a GL 1.4 site or using a beta release of GL 1.4.1, then update style.css file using the pre1.4.1_forum.css file in the archive under the themefiles folder. Backup your current style.css file for reference and for many sites, they can just replace all the plugin like CSS declarations. They should all be grouped together in your style.css file.
* Pre 1.4.1 themes will also need to add missing navbar images. Add the images found in archives themefiles/navbar/images folder to the same folder under your site's theme.
Add the CSS declarations from the forum.css file in the archive under the themefiles folder.
* If you have a GL 1.4.1 final site, then you will only need to add the few additional CSS styles for the codeblock and quote formatting.
Add the CSS declarations from the forum.css file in the archive under the themefiles folder.