It's really the opinion of the audience thta makes the difference. It is influenced by a lot of in-story attributes such as the general behaviour of heroes and villains.
"For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." - Richard Feynman
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Yeah, but what behavior depending on the Hero or the Villain would for example make you feel that they are going too far into one end of the scale?
Well, if the heroes have a habit of breaking stuff and engaging in villainous behaviour while the villains are reeeeeally proactive in doing damage, I would consider the work to be "dark".
"For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." - Richard Feynman
No, the other one.
All of these concepts can be used together in various degrees. You have have a silly story that's also dark and edgy, or a serious yet light-hearted story. There is no real line separating then. You can have something that takes itself so seriously it's silly, either by accident or deliberately. You can flip between them as much as you want as well, for better or worse.
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Sorry about being so slow but can you please elaborate a bit more?
A silly story taken seriously would be Epic Mickey right? While a serious yet lighthearted story would probably be Avatar: The Last Airbender (first season) right?
No, the other one.
Not really sure about Epic Mickey since I'm not that familiar with it, but it does have a few contrasting elements in the presentation.
Avatar: The Last Airbender is overall lighthearted and occasionally silly, but does for the most part take itself seriously, and like most stories it gets darker over time (past the first season), though I wouldn't really say edgier. Serious and handles more mature themes, but not really much grittier or significantly less idealistic. It does cover a relatively large span, though. It's really both silly and serious.
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Ok thanks. 
What is the fine line between something being so far into the Darker and Edgier and going so far in the "serious" end of the Sliding Scale Of Serious And Silliness that it becomes ridiculous....
Ex: Loonatics Unleashed took itself way too seriously but I heard that it stopped doing that in the second season.
...and something being Lighter and Softer and going so far into the "silly" scale that it you feel like you're losing brain cells by watching it...
Ex: I believe you can come up with some examples.
Basically, what I am trying to say is....what is the line you cannot cross between making a "Dark" or "Light" story? What point can you not cross to keep it from being ridiculous? How can you tell which tone will work based on the concept?
That's just a question that's been on my mind.....