I would love to have more control of colours instead of just a handful of themes. I already up-voted a few specifics, like the iCal colour choice, which is a make it or break it feature for me. I have 10 different calendars importing from iCal, and they're all purple. But overall, having more control of the appearance would be great. So many freelancers are creatives (and neurodivergent if we're being honest), and we need colour not just for aesthetics, but for functionality.