Hi there , here is collection of icon replacements I made for apps not yet updated for macOS Big Sur. For instructions on how to use them check the bottom of the page.
Text editor made by Panic
Great 2FA App with iCloud Sync
The easiest method of changing your icons is with Pictogram. This way your icons stay modified even after updating them. This does not work for System and AppStore apps tough.
If Pictogram does not work there is another way to change your app icons:
1. Right click on the app in the dock → Options → Show in Finder.
2. Press ⌘I (Or right click → Get Info)
3. Drag the new icon onto the small icon in the top left of the Info window.
4. Done! Keep in mind that this will reset with every app update.