Hi Oyvind:

All the apps utilize view templates that are fully customizable like Ermin mentioned. You may want to refer to the following documentation about Views to get started.

https://www.odoo.com/documentation/13.0/reference/views.html

You can easily identify the views to be customized by going to the forum page on the website as an administrator and clicking on the Customize > HTML/CSS/JS Editor menu option. You will need to browse through the views listed in the dropdown list on the right to identify the view(s) to be customized.

Hope this helps in getting you started. Welcome to the wonderful world of Odoo customization!

So, this should be a comment to Ermin, but missing karma won't let me comment. Interesting...

OK Ermin, that's good to know! Could you please share how I can start customizing it? "Edit" in the top right lets me change the page, but nothing on the forum app.

The forum app is an app as any other also. You can customize it like any other app.