The best way to see what CSS controls what is to turn off css caching in the backend. Then you can use the inspector plugin of your browser to look at the code and it will show which css classes or ids are used.
If you want to add custom classes and ids, then you need to study the template system. The template system uses keys and html files to control the template of the site. These are not regular html files; they are parsed by the template system to help generate the actual pages.