Recurring prompt to confirm Style Sheet when opening Safari - How can I get rid of it?

Every time I open Safari, I'm asked to choose a style sheet. I haven't ever chosen one. I've tried to find one it suggests (webguard.css from an old application long removed) but it doesn't exist on my system.

I've done the Safari/Preferences/Advanced/Style sheet setting with none selected but it doesn't work. It reverts every time to the same old question.

If I cancel, it works fine until the next time I start Safari.

I've referred this to Apple who have been through the whole diagnostics process and can't find anything.

Ultimately, it's not a show stopper, but where can I find the place to remove any reference to a style sheet in Safari?

One solution might be selecting a css file that does exist. Any plain text file with a .css extension should be fine. This might fix whatever setting is corrupted.