Both of them, as for me.
Colors are RAW black - it is impossible to work much time with such them. Grey or bluerish-grey do not hit in the eyes as much.
Gigantism also not good. There are too much preferences, that might be checked before printing, and with such big spaces, you'll have to scroll too much, instead of viewing all of them at ones.
If it possible and developers have time, just make extra theme. They know code, for them it takes 1 hour, for us - it takes 1 day. Because there is no autoupdate, you have to reopen Cura to show changed effects.