The default icons have a bit depth of 8bit. As soon as you switch the game to a color depth of 32bit, they end up black.
You can fix this without using a paint program by right-clicking the sprite in the Sprite manager, copying it to the clipboard, then overwriting it with the clipboard contents. This will cause the sprite to be re-imported at 32bit and it should appear properly afterwards.
So it's not really the Default game's fault, it's just that its resources date back to the DOS era.