A good ST series will have its share of "dark" episodes as a natural outgrowth of the characters, setting, etc - along with light-hearted and humorous material to balance it out. TOS and TNG had this, but I think DS9 did the best job of any ST series in balancing the dark and light stuff without getting too far into either.
I don't really think it works to compare ST to BSG as they are vastly different shows that really have nothing in common other than being set in space. BSG was dark just because of the basic premise of the show.
Don't worry about making a "dark" or "optimistic" or "dramatic" or "gritty" ST show - just worry about making a good show and let the other adjectives fall where they may.