In this page you can manage all your themes or add a new one.
You can view in the right this panel, in the textbox you can paste a theme info.json link (If you want know more about info.json, view in the development guide) and when you click add, mycms download it.
Like you can see in the screen there are three themes. The default theme can't be deleted.
In the theme info page you can view which files and folders are in the theme. In the right you can view three button and the theme info.