Head of Product for Zulip here. We appreciate the feedback! We are actively recruiting for a designer role (https://zulip.com/jobs/#designer), and making these types of improvements will be a major priority in the coming year.
I agree with the OP, but I have to say that your keyboard navigation is second to none. Also, please keep Zulip fast, it's the biggest reason why it's so great to use.
I saw that! in the commit stream during today's upgrade. I'll riffle my network - design is always tough, and designing for Zulip is esp tricky given its extensibility.