I do agree that Zed's default themes aren't great. They look too 'plain' for my taste. Bit more contrast can't hurt either.

BUT: It's very easy to just choose a different theme and there are plenty to choose from by now. It's even possible to make your own theme and they even have a first-party theme editor (https://zed.dev/theme-builder) which works great. They should maybe include some descriptions for each color instead of just the name but that's the only negative thing I can say right now.

I'd even say that it's easier to theme Zed than VSCode because there are fewer variables.