I'm just playing devil's advocate here; I don't necessarily personally agree with some of the things I say in this comment.
Let's not be too hasty to accuse people of illegal or unethical activity. Consider the possibility that stealing the design was an idea an employee or contractor had on their own, and the founders were unaware. Or maybe both companies bought the design from the same third-party provider. Or maybe one of them developed the design and legitimately licensed it to the other.
> How can the co-founders morally justify that it's OK?
If the victim wasn't okay with the site's styling being copied, they shouldn't have made it publicly available. Besides, CSS is trivial and therefore not something all that important.
> Aren't they afraid of getting sued like how Touch of Modern was sued by Fab?
If Lot18 is a small startup, they might not have the resources to sue. If Caskers is a small startup, maybe they don't have enough assets to be worth suing. If they're both small startups, maybe the intersection of their userbases is sufficiently small that there's a high probability no one will notice.
> Were they hoping people wouldn't notice?
I'm sure that was the ideal outcome, if someone did in fact steal the design.