Note to Stardock: Please, please add a dark mode and modernize the preview window. That’s the only thing keeping this from a perfect score.
IconPackager cannot change icons for Microsoft Store apps (Calculator, Mail, Spotify). Those live in protected Appx containers. If you want a unified look, you’ll have to hide those icons or use a separate launcher like Rainmeter.
You can build your own icon sets using PNGs or ICOs. Drag and drop 50 images, map them to specific shell objects (e.g., "Folder - Open," "Drive - SSD"), and save the package. The batch conversion tool is surprisingly fast, resizing 256x256 PNGs to proper .ico formats in seconds.