To fix this,
Make sure civil 3d is NOT running.
Go to C:\Documents and Settings\
(YOUR USERNAME)\Application Data\Autodesk\C3D 2010\enu\Support and make a copy of c3d.dll and rename it so that it has the same name as your custom cui, (which should be in this directory by default), but with the .dll extension.
Start Civil 3d. Icons should be fixed.
You may need to switch workspaces and/or tab.
In part of the help file it says that icons are sourced from icon files including .dll.
So putting it together, I worked out that the c3d.dll held all the icons that civil 3d uses.
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Andrew Puller
Maitland, NSW, Australia
Windows 11
Intel core i7 11800 @ 2.30 GHz with 32GB Ram
Civil 3d 2023