I'm looking for a specific cartoon TV show from 00s about kids possibly living in a...quite big house?

What I remember from this show is there were a bunch of human kids and a biiiiig house, and there were also monsters/creatures of some sort. I remember the color palette from my mental image is reddish-blackish, so darker colors. From what I remember it was a kinda dark kids show.

Other info I can give you: It was potentially on Nickelodeon or Cartoon Network, or maybe Fox Kids/Jetix or Discovery Kids, because those were the channels I had at the time. It could be an older cartoon that was shown in the early 00s, but it looked newer than most of those.

Things I’ve discarded so far have been Creepie, Aaahh!!! Real Monsters (too old and…realistic), Foster’s Home for Imaginary Friends (too recent as well), Invader Zim…

If it wasn’t for my sister also recalling the show I’d think I was imagining it. I also remember I found the intro or a video from that show a few years ago looking for cartoon intros and then…forgot the name again. Oh well. If any of you can figure this one out, you deserve a Nobel Prize.

EDIT: I’m now remembering two things that could help. 1, it could have just been a school and not a house, and the reason it was dark was it was an attic. Not sure why I only remember the one scene. The kids kinda looked…gothy, as well?
And 2, I believe the whole reason behind there being monsters and stuff there is they were solving mysteries.