California falls back on Spanish civil law. The most obvious example of this is the fact spouses cannot hold property individually in California, except for a few (constitutionally protected) items.

In general, each sovereign jurisdiction will fall back on the legal system that was in place at the time the jurisdiction gained its sovereignty. For California, the ancestor state was Mexico, and thus Spain.

https://www.law.berkeley.edu/library/robbins/pdf/ca-legalher...

But to answer the larger question. Everything that is not specifically legislated against is legal. Some ancient laws may still hold, but most are probably going to be found unconstitutional. Although, there may be some exceptions that you could convince a court to enforce.

Well, state law could fall back on martian (with an "n") law. It doesn't matter. Laws exist to state the thing you cannot do. If there is no law against the thing, the thing's not illegal. Falling back on another system doesn't make it any less true that unless there's a law against a thing, you are free to do that thing. And I'd add that laws stating a thing must be done a certain way are simply excluding all other ways of doing a thing. And even if English Common Law fell back on some unspoken Viking Code, the entire body of law exists to state what is forbidden.

The English allow sports betting, don't they?