I haven't finished it yet, but I really liked Keep Your Hands Off Eizouken. It's about three middle schoolers trying to make an anime. It's comedic and a love letter to animation production. Also Haikyuu!! I haven't caught up to that either, but I can't imagine that anime getting into anything dark. It's normal dudes playing volleyball.
Like someone else said, it really depends on what your idea of what 'dark and heavy themes' is. Mob Psycho 100 is an anime that I'd say isn't super dark or heavy thematically, but it has moments that are.
If you want recommendations that aren't dark or heavy at all, comedies, slice-of-life stuff, and various things aimed primarily at children are generally what you're going to get.
@river121693 Also seconding Azumanga Daioh! I haven't seen K-On! but it looks like it fits the bill. Lucky Star probably also counts. Nichijou as well.
Not strictly a comedy anime, but I also remember liking Princess Jellyfish.