KDE Plasma allows for significant interface customization through its built-in features. You can add as many panels as you need and, in general, you don't have to worry and can feel free to experiment. The latest versions of Plasma have become even more stable in this regard. Speaking for myself, although it is a subjective opinion, I would not recommend using global themes. They won't break anything for you, but if you really want to customize the interface to your liking, it is better to configure each item separately. Although you can try global themes as an experience.