Season 4 was an attempt at this after a particularly dark season. Oliver finally adopted his comic-book moniker of "Green Arrow", along with a lighter shade of green for his suit, and declared his intention to be a 'symbol of hope' for the city as opposed to the ruthless vigilante he was as the "Arrow". Oliver was in a relationship with Felicity and generally had a more optimistic and agreeable demeanor (dubbed "Ollie 2.0" by Thea). Team Arrow had a swanky new lair that was literally brighter and was more of a family than ever.
In season two, Oliver decided to stop killing criminals in order to prove that he was better then the criminals that plagued the city. This after season one, where he killed a significant number of his opponents, and in the end he manages to stop Merlyn's earthquake machine... only for the back-up machine to go off and cause a great deal of death and destruction anyway. Season two's still a bit dark but an Oliver who only kills as an absolute last resort has much more of a victory against the bad guys.
Season Seven: After Oliver gets out of prison, he starts trying to do everything in the light of day and through lawful means, being deputized by the SCPD as what is effectively a black ops unit of the department and appealing to Emiko as the hero that he knows she could have been if she had a better upbringing.
The entire series is like this compared to Arrow, with more humor, and the characters still having some Soaperizing but not to the point where things are extremely strained all the time. Even noted in-universe, as The Cowl Oliver tells The Cape Barry that Barry could inspire people in a way that he himself never could. Barry's optimism and willingness to do the right thing even if it seems victory would be better assured by playing dirty is considered one of his best qualities, and is contrasted to Oliver even by Oliver.
There's the Flashpoint reality. Elsewhere, Flashpoint and its Animated AdaptationJustice League: The Flashpoint Paradox are about the world becoming a total Crapsack World with global destruction imminent due to the war between the Amazons and the Atlanteans (with Aquaman and Wonder Woman as competing Big Bads in a war over who will Take Over the World. In the end, everyone loses.) In the show, the episodes "Flashpoint" and "Paradox" show the altered reality to not be so bad. However, that makes restoring the old reality once Wally is mortally wounded in battle against The Rival much more of a sacrifice.
Season 4 has a lighthearted and comedic tone after the darkness brought by Zoom and Savitar.
The show is produced by the same team behind Arrow and The Flash, is notably less serious than either one of those shows, with the prospect of superheroics being treated in a very fun and free-spirited fashion, without as much major emotional baggage that the other two shows have had to deal with. In much the same way, it's also less serious than the last live-action iteration of the Superman mythos, Man of Steel.
The Freeze-Frame Bonus mentioned below gives us an In-Universe justification for the tone, as well - while hardly utopian, the reality Supergirl occurs in appears to be a good one.
That said, it's lighter and softer with teeth, as the DEO - especially Kara's sister - often use deadly force to bring down villains (a heartbreaking example when Alex is forced to kill Astra) and a couple of episodes have addressed whether Kara is capable of doing the same if necessary.
Legends of Tomorrow is an odd case. Content-wise, it's got a lot of dark content that wouldn't fit on The Flash (2014) or Supergirl: the Main Characters frequently kill people, several of them used to be murderous criminals, and the show readily acknowledges that they often do just as much harm as good. However, tonally, it's easily the lightest of the whole bunch: most of that dark content is Played for Laughs, and the stuff that is played seriously will still have some comedy to leaven or undercut it.
