The default themes, also known as generic XAML files, contains all styles, templates, and brushes used to render all Ultimate UI for WPF controls. You can use these themes as a base basis to create your own themes, or you can just pick a few specific styles and modify them in your application. By default, the installer for Ultimate UI for WPF controls installs default themes in the following directory:
C:\Program Files (x86)\Infragistics\2023.2\WPF\DefaultStyles