This is a special program, built on top of content_shell and incorporating a multitude of options for testing, viewing, and debugging UI resources in Thorium.
Run the thorium_ui_debug_shell.exe (Windows), or Thorium_Debug_Shell.sh (Linux), and you can select from the menu. Some things are interactive, some load internal resources, and some require loading external resources like viewing .icon files. In that case, you can load a file using its full path in the box towards the bottom.