I'm guessing you are referring to
There are two ways to modify Discourse in ways not presented via the Admin UI
* A lot can be achieved using the Admin UI so IMHO you should look into what can be done there if you haven't already. eg.
But if what's there isn't enough, you can:
- Hack the Core code
- and maintain your own Fork
- reapply your hacks every time you upgrade
(read - Maintenance Nightmare / Security Risk - so please don't)
The above linked SitePoint theme is a plugin.
To install it please see
* Note, Discourse uses SCSS. so if you are wanting to make your own tweaks you should be somewhat familiar with it though CSS alone will get get you far