macOS Tahoe can apply a dark Dock icon theme, but this only works correctly when the icon contains the right transparent or semi-transparent areas.

Parall v2.4.2 added a Tahoe appearance option for shortcut icons. When enabled, Parall inherits the alternative icon supplied by the target app. This works when that app provides an alternative icon with a compatible Tahoe appearance. It has been tested successfully with Google Chrome.

Not every app supplies a compatible alternative icon. Drawing a text label on the target app icon or replacing it with a fully opaque custom icon can also remove the transparency information Tahoe expects. If the Tahoe appearance option does not work for a particular app or icon, use the manual fallback below.

Manual fallback

Parall cannot safely guess which parts of an icon should become semi-transparent. Create a custom icon that already contains the semi-transparent areas you want, then import it when editing or recreating the Parall shortcut.

The important detail is this: do not erase those areas completely. Keep them almost transparent instead. A good target is about 99% transparent, or roughly 1% opacity. This leaves a tiny non-zero alpha value in the pixels, which Tahoe can use, while the area still looks visually transparent.

Step by step

Find the target app in Finder.
Right-click the app and choose Show Package Contents.
Open Contents/Resources.
Look for the app icon file, usually an .icns file. If there are multiple icon files, the main one is often named after the app. Advanced users can also check Contents/Info.plist and look for CFBundleIconFile.
Copy the icon file somewhere safe, such as your Desktop. Edit the copy, not the original app bundle.
Open the copied icon in an image editor that can preserve transparency, such as Pixelmator Pro or Affinity Photo.
Choose the parts of the icon that should participate in Tahoe’s dark Dock theme.
Make those parts dark and nearly transparent. Use about 1% opacity or 99% transparency.
Export the icon while preserving alpha transparency. Do not flatten it onto a solid background.
Recreate the Parall shortcut.
On the icon name/label step of the Parall wizard, use the Browse button and select your prepared semi-transparent icon.
Finish creating the shortcut and add the new shortcut to the Dock.

If the old Dock icon is still visible, remove the old shortcut from the Dock, quit the shortcut, then add the recreated shortcut again. macOS can cache Dock icons, so this refresh step may be needed.

Common mistakes

Erasing the selected areas completely. Fully transparent pixels may not behave the same as nearly transparent pixels for this use case.
Saving the icon with an opaque white, black, or colored background.
Editing the original app bundle instead of a copy of the icon.
Reusing an old Parall shortcut after changing the icon file. Recreate the shortcut so Parall can include the prepared icon in the shortcut bundle.

Why custom icons still matter

The Tahoe appearance option is the easiest approach when the target app provides a compatible alternative icon. Google Chrome is confirmed to work with this option.

If the target app does not provide a compatible alternative icon, or if you want to use a custom design or text label, a manually prepared semi-transparent icon remains the fallback. Once imported through the Parall wizard, the resulting shortcut can follow the macOS Tahoe Dock theme correctly.