All icons are white in MacOS 26

Odd! On my Mac with Tahoe, Excel file aliases are the generic green Excel icon, Word is blue and PowerPoint is orange. PDFs and screenshots have the image preview as the icon.

Have you tried shutting down your Mac then starting it back up (different than a restart)? You can see it the white icons are present when you boot in safe mode, which disables 3rd party extensions and performs some system cleanup.

Start up your Mac in safe mode - Apple Support

I tried playing around with the contrast and other settings in Accessibility > Display but I couldn't get the desktop icons to change (except to other shades of their original color).

I was having the same problem and went into Show View Options to see what was there. I unticked the option for Show icon preview and my Word and Excel icons changed back to how they were before. Worth a look.

That setting controls application icons in the Dock, not in Finder. Document icons are set by the application. In many cases, the icon is a preview image of the document, so a Word doc and its corresponding pdf will look the same. You can use the Kind column to tell them apart.

However, there does appear to be a bug in Tahoe, in that for the List View I clicked the Name column header to reverse the sort (and then clicked it back), and the icon for the original Word file changed to the default Word icon instead of the document preview image. But maybe that 'bug' will help you while it lasts.

Robert:

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?

The settings in Appearance control folders but not icons, those are set based on the application. Perhaps sharing a screenshot would help (don't include any personal info).

Take a screenshot on Mac - Apple Support

Try if fiddling Icon size via Finder > View > Show View Options helps.

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?

I'm experiencing the same issue and have yet to find a work around. My Microsoft document icons display identical to previous versions of Mac OS. Generic icons such as PDF and MOV file icons are blank. Text Edit document icons are also blank. I conducted a finder search for all files with the .pdf file extension, selected all, pressed "Get Info" to all, then copied a sample Apple Preview PDF icon into the top showing info panel, hoping to execute a batch change. Oddly, the batch change did insert the standard Preview PDF icon for half but not all the files. I have approximately 5,000 PDF files in my system, converted from various sources such as Word files and Safari windows, so perhaps it succeeds or fails to convert the icons based on the original source. I'm just guessing here. All icons from newly created PDF and MOV files continue to show as blank. Go figure. Because I use the icons to quickly identify file types, the blank icons are a serious obstacle to a smooth work flow.