Does anyone know how to make a level dark like in one of the levels of world 4 of NewerSMB?

That's basically done by replacing any of the existing FG effects (like the clouds or the snowflakes) with an entirely black color and reducing its opacity, the FG effects can be found in the polygon_unit folder, you can either replace an existing one or an unused one.

Also, if you're gonna attach images to your post, you need to post them on the uploader and copy the link and paste it in the middle of the "img" tags, otherwise your image won't show up in your post.
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Newer ds does it via custom code, it lowers the screen brightness, even the coin counters get darker, its just terrible, but yeah use what ztar said, it will be WAY better :3
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Note though, FG effects do not affect 2D sprites, so this method will look pretty strange if you plan to use Goombas, Koopas etc.
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