Well, there are some considerations that you need to be aware of to fully change the design - Template, Styles and Builders.

The template you choose will give you a certain navigation layout. Then you can go into that templates Styles section and create your own style "set" to change all the fonts, colours, shadows, margins, borders, etc. You can save/export/clone your sets or roll back to the default one - so that's not disruptive and good for experimentations. 

Then, you might want to think about your choice of icons and menu styles for various modules and system pages submenus, action menus, etc. There are hundreds of them, so generally, you just leave them as is. They're using Fontawesome for icons and you can bulk-change the FA style if you have the FontAwesomePro app installed (part of UNA Pro and UNA Cloud plans). 

Mind you, the template is not "aware" of the modules and submenus navigation items texts and icons per se - so you change them in Navigation builder, where you can upload custom icons, change items texts and select styles for each menu (vertical, horizontal, text-only, large icons, etc.). 

Once you go through these, you'd see the reasoning behind the structure.