Dark Shadows has a narrated introduction about how important family is, and there's a takeaway about avoiding the "curse" of a loveless life, but these themes aren't conveyed strongly. Basically, It's a one-man show, with Barnabas taking center stage and rarely turning to his family or helping them with anything. The other family members appear to despise one another. An uncle chooses to leave his son in exchange for money. And in general, problems are solved through violence.
The movie's mood is comic, so none of the violence has a heavy impact; but there are also no notable consequences for the killings -- and there are a lot of them, with lots of blood (which seems to have been deliberately colored to resemble the fake-looking TV blood of the 1960s). Most characters die off screen. The movie's climax includes a special effects-heavy supernatural fight between characters (including a stream of green vomit) but little brutality. In a flashback, parents ship a little girl off to an asylum because she sees ghosts; there are some potentially upsetting shots of her locked up in a cell and receiving electroshock treatment. Also some spooky ghosts and an explosion/fire.
Plenty of innuendo (references to touching oneself, making noises, etc.) and some passionate kissing, but no nudity. Barnabas kisses or has sexual encounters with three women (four if you count that the same actress plays women of two different eras). He has supernatural "sex" with a witch: Locked in an embrace (and still clothed), they crash all over the walls and ceiling of a room. A female character lowers her face, off screen, to the main character's crotch to (presumably) give him oral sex. A woman rips off her top to reveal her cleavage and places Barnabas' hand on her (covered) breasts. Also of note is the way in which 15-year-old actress Chloe Grace Moretz is sexualized in her scenes, posing or dancing sensually or reciting dialogue in a sexy way.
"S--t" is used a couple of times; "bitch," "damn," "a--hole," "hell," "bastard," "whore," "oh my God," and "balls" are also used. There's also a range of insults, including "stupid," "harlot," succubus," etc.
When Barnabas emerges from his coffin, he's entranced by a giant "McDonald's" sign. There are lots of references to 1970s-era products, some of which are still around today -- there's a Wheaties cereal box, toys like Operation and Rock 'em Sock 'em Robots, and a "troll" doll. Scooby-Doo is shown on television.
Drinking, Drugs & Smoking
A supporting character is shown to have a drinking problem. It's treated comically, but she's drinking and belligerent in nearly every scene. She also pops some kind of prescription pill. Another supporting character is shown slobbering drunk in one scene. Barnabas is around a group of hippies who smoke pot. A 15-year-old girl is involved in a joke about being "stoned" (shown in the trailer), in which Barnabas misunderstands the meaning of the term.