Custom Safari toolbar icon greyed out after migration

Got a new Mac, used the Migration Assistant thing to transfer everything over to it from my old one. Generally everything's worked beautifully, but the custom Safari toolbar icons are greyed out and nothing I do seems to change this. I've tried dragging them back off, emptying the cache, dragging back; no joy. It's not the biggest deal in the world but it is bugging me a bit and I'd quite like to have those icons there if I can. Any suggestions gratefully appreciated.