Martin,
Hands up - The black icon is my fault. As you know it's quite a complex system under the hood, indeed the configuration seems to be duplicated at a global and local level and somewhat confusing in the abscence of instructions.
Yonks ago to determine what files I was looking at/ what level I was at I changed a CSS file to make the button a different colour temporarily. Bob's account accessed the system whilst the #hack was active.
I changed the colour back but the cache on Bob's browser is/ must be still storing the old temporay CSS
Sorry...
C-3PO