I find the way to manage styles not very intuitive. When I right-click on a style, for example to delete a symbol from the style, and click on "Manage style", it opens the catalog. On the left, there are all styles, and on the right, I can access the style classes (though it took me a while to find it).
However, there is no easy way to switch between point, line and polygone symbols in the style - you have to go back completely and open the style again and then you can choose a different style class.
Altogether, I find the style manager pretty clunky and not very easy to work with.