I have a silly question- if you like dark beers, why don't you make a dark beer instead of buying a kit and adding molasses? Maybe it's because I don't like molasses, especially fermented molasess, but if you like stouts, why not just make a stout? If you get a stout, built to be a stout, without corn syrup or sugar, I think you'd get a better beer.
If you like brown ales, then there are lots of nice recipes for brown ales. I'm not a fan of buying a kit and adding sugar to get it to be closer to what you might want. You can buy all the ingredients you need for a GREAT beer, in the style you want, and have better results.