I’d actually say that it’s quite important to watch from the very beginning, not because you’ll miss anything otherwise (as noted in the 'cast, yeah, they’re quite good at helping viewers catch on) but because it’ll ruin some of the surprises.
And with that in mind, I wouldn’t say that Eureka is never dark and broody, just almost never overtly so. Much of the ongoing plot in Season 2 was often not directly mentioned, if I recall in some episodes it wasn’t explicitly mentioned by any characters at all, but the actors were really good at playing it so that you can just barely glimpse what’s going on inside their heads. I remember watching Season 2 and finding it often both lighthearted and quite dark simultaneously. Obviously I’m being pretty vague here since I don’t want to spoil it for anyone who hasn’t watched it all yet.
You can get it 