:3 Oooh I think you'll love CLAMP then~ They're fond of the whole "start out sweet, then suddenly get serious".
Both Cardcaptor Sakura and Magic Knight Rayearth are magical girl series [though Rayearth mixes mecha]. CCS takes place in the present and doesn't really get dark ever. Rayearth zaps its three heroines into a fantasy world and, I've heard, does get darker as it goes on though I'm not sure how significantly. Both contain magic! 8DDD
Tsubasa goes from light-and-silly to omgdark. However the story, as much as I adore it, is poorly paced. The characters are lovely but the cheerful first half is too long and the middle is emo, not to mention the super-confusing ending. I'd give it a shot--read a chapter or two and if you like any of the four main characters [Sakura and Syaoran from CCS also star as leads here, though older] then stick with it. If none of the four catch your interest it will be a rocky ride. :C
XxXHOLiC is a light-then-darkens series. It is very interesting~ Modern day spirits story. Sister series to Tsubasa, and much better plotted.
X is dark-then-omgdarker. Apocalypse and whatnot. Warning--shonen-ai friendly [well all of CLAMP is, really]. Tokyo Babylon, it's older sister series, is actually shonen-ai and is light-then-extreme!dark.