A couple of free Tapas comics come to mind.
First comes the ridiculously underrated treasure, Children of the Moon. Concept: Cats see the invisible creepies lingering at the edge of our reality, so when reality-breaking horrors from beyond space threaten the world, cats find themselves recruited as the world's first and only line of defense. Theme: In an age when the term Lovecraftian gets thrown around too much, this comic really delivers on a genuinely Lovecraft-like read. It's also gorgeously illustrated and fully colored.
The Lost Graveyard is a brand new arrival here on Tapas. I read the first episode yesterday and thought it was a very strong start. Concept: six strangers crawl out of their graves one night. They don't remember how they got there, and they cannot leave their small graveyard. Theme: The look is stylized and very dark, like a Gorey illustration. I look forward to a lot of mystery and character-derived plot twists.