How would you make a light beer taste more like a dark beer? In other words, if you like light beer better than dark beer but you think that light beer could use more taste, what could you add or is there such a beer out there?
It depends upon what you think a dark beer tastes like because there is a wide range of shades of dark, and a wide range of flavors that come with dark. The amber beers can have tastes of caramel and dark fruits like raisins. Stouts, like a Guinness, will have roasty flavors, and those and porters can have hints of chocolate and coffee.
The easiest is probably to add a little dark beer to your light beer, like you have with a half and half. The downside here is that you open two beers when you only want to drink one, but you could always save the mix-in beer in the fridge. You could also add come coffee, a tablespoon at a time, and see if you like it. Or, you could cut it with, not a dark beer, but an amber colored one like an Irish Red, or a Mexican lager like a Negra Modelo, if you don't want too much of a "dark" flavor.
If you really want to do it "right" and you live near a home-brew shop, pick up some caramel malt, like a crystal 40 or 60, make a little tea with it in some warm water, and add that to the beer. That's basically what goes on in the brewing process.