Horrible / Horrid?

[museumum]

Are they synonyms?
Where and when I grew up "horrid" was never used - it sounds a bit "famous five" to me (dated/English/posh)

If you use both, what are the rules for when to use each?

I use both but I agree that horrid is a bit Enid Blyton. Horrible is more neutral.

They're kind of variations on a theme, aren't they? So usage depends on context. I would only use horrid in a "slugs are horrid" spoilt brat kind of context.