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I'm using "The Johto Elite" as my go-to dark skin. It's pretty easy on the eyes and has no "unnecessary CSS" (though I'm not entirely sure what you mean by this). If you don't like the banner, go to User CP > Edit Options > Forum Style display mode (at the very bottom of the page) > Reduced Mode, and the banner will disappear. :D
If you're using the Firefox browser, why not download the Ad-block add-on, which, when installed, permits you to disallow images you choose, that way you can disable banners in which bother you. As for "Unnecessary CSS", there isn't a great deal of that around the forum, so I'm not sure how I could help you there. But as for dark styles, I recommend trying the following skins:
Or you can do as aforementioned and attempt at looking at some of the styles yourself via your user CP or the drop up box on the bottom left corner of the forums which allows you to choose your forum style.