Besides the color palette, it will be CSS/SCSS, and more if needed (JS, HTML).
You might be interested in this:
You can create themes and components. Components are like “subthemes” (they can contain styles and features), and any component can be enabled for one or multiple themes.
There’s a command line tool that helps for creating themes and components and I suggest you have a look at it:
There’s also a special page that lists most Discourse HTML elements so you can have an overview of what they look like, and create your styles accordingly.
The page must be enabled with the Styleguide enabled site setting.