A lighter tone speaks of brightness, rich, glowing, luminous, and clear, while a darker tone presents gloomy, dull, shadowy, harsh, strong. In my opinion, lighter themes would go best for an online business website. Lighter themes seems to be welcoming compare to a darker theme which I presume to be edgier, inviting younger demographics. If I assume to be a client or a customer, I would visit a website that has lighter theme, for the reason that I presume the owner to be as transparent as his website is. If what you are after is an online business suitable for any generation, you should go for a lighter one.