Hallo Tim,

you have two options to style a course in the AuthoringTool, a custom Theme or custom CSS added to each course directly in the AuthoringTool.

A custom theme is great for applying the same styling to a large number of courses. You must however create your own Theme (based on vanilla for instance).

Custom CSS added directly in the AuthoringTool is great for course specific styling. You may overwrite certain LESS-Variables for instance. You must however apply the styling manually to each course. This can become hard to maintain for a large number of courses. Thats why we recommend the usage of custom Themes.

Both soultions require basic knowledge of CSS / LESS. A good starter is this page: https://github.com/adaptlearning/adapt_framework/wiki/Styling-your-course

Hope this is useful, you may drop me a PM if you prefer talking in german. 

Cheers

Thomas