I can see everything clearly with both of my monitors at my office and at home with default contrast/brightness settings.

I guess our monitors behave very differently then. I don't know maybe my LCDs are too bright.

I guess my monitors not callibrated correctly then I can see it on my lowest brightness setting ::/

 No, not every single one has to, but I wasn't even suggesting at being very "bright or contrasted" -- I was suggesting it to have a bit more contrast and brightness to the point where I don't have to turn my monitor to the top brightness level to see any detail (or really anything at all).  Doesn't need to be flamboyant -- just visible.

I think the darkness is good as it is. Not every pixel art has to be bright and contrasted. In my opinion you don't need to change anything.

 It's very dark, and more so than that, all of the colors have a similar chroma therefore don't have any real constrast against each other, besides that provided by the differing hues.  Definitely make the skeleton guy much brighter, along with his glowing effect.  And while it's hard to notice with such a low color contrast, most of the bricks seem to be simply a blob of a solid color surrounding the skeleton dude -- after a color change, it doesn't look very good at all, so after working with colors I would fill in more of the bricks behind him.