The Kunena forum you can add different class for each category and you can style for that class. In this article we'll research "How to add CSS class suffix for each category?"
Step 1: Login to your back-end of your site and go to the "Components / Kunena Forum / Category Manager".
Step 2: Click to a category you want to add the CSS class suffix. In the "Category name and Description" tab you look down at the bottom of page. You'll see the "Category CSS class suffix" option and you type the your class name.
Step 3: Click Save button to save your configuration.
Note: You need add the a space before the class name. Example: [space]class_name