The original discussion stylesheet for all portions of the discussion application is located at
packages/discussion/css/discussion.css
To modify this file in an extendable way, copy it from this location into the root of your current theme. (e.g. if your current theme is "mytheme", copy this file to themes/mytheme/discussion.css and make your changes there.)
The discussion addon comes with two page types, Discussion and Discussion Post. A discussion acts as a "forum" or "category" page might in another forum model. To create a sample forum, do the following.
1. Create a page of any type that is NOT discussion or discussion post (so right sidebar, full, etc... or any other custom page type you've added to your site.)
2. Add a Discussion Categories block to this page, with options set to display discussions that fall below it.
3. Add a Discussion page below this one. Once approved, it should automatically show up in the discussion categories block that you added in step 2.
4. That should be it. At this point, the discussion page type should allow new posts to be created under it (by adding a topic) without using the concrete5 controls. These posts will then be displayed by the Discussion Topics block, which should have automatically been added to the Discussion page you created in step 3.
The page type templates themselves may be customized as well. This takes a couple steps. For example, to customize the discussion page type, do the following.
1. In your currently active theme, duplicate view.php.
2. Rename this file to discussion.php.
3. Copy the contents of packages/discussion/page_types/discussion.php.
4. Open the discussion.php file you created in step 2.
5. Find the line <?php print $innerContent?> and remove it. In its place, paste the contents you copied in step 3.
Now any edits you make to the discussion template in your theme should be reflected in your site, without having to modify the core discussion application.