Get your buzz, light or dark roast
Light and dark roast coffees have very similar caffeine levels. Although caffeine has a boiling point of 352oF, it can generally withstand the dry heat of roasting temperatures far exceeding 400oF. If there is any loss, it is negligible amounts in dark roasts. At Just Us! we only roast green Arabica beans, which contain about 1.2% caffeine. Robusta beans, the other commercially cultivated species readily used in commercial blends, with less flavour and more bitterness, can contain close to double the caffeine of Arabicas.
Although there is overall weight loss from loss of moisture in the roasted beans, each bean still has close to the same amount of caffeine as when it was in the green state. If you weigh out your roasted beans for brewing, you will use slightly more beans per weight of dark roast because it loses more moisture in the roasting process, but again negligible amounts. So any slight difference in caffeine between a light and dark roast would essentially equal out with the slight difference in beans used to brew.