I wish I could understand the logic behind modifying a user interface that worked perfectly well. Things are so bad that I'm considering switching to another calendar provider.
I complained a while ago about the lack of contrast for past events, which makes them very difficult to read (something I need in order to do my billing), and was told that this is something your developers are working on.
Today, on a whim, I switched to dark mode, and that helped alleviate the above issue, but dark mode requires a bit more thought put into the design than just making the backgrounds black.
As I said, past events are more readable thanks to the added contrast with the page's background (it's an optical illusion, but it works).
When I click on an event in light mode, my eye automatically goes to the detail pop-up:
In dark mode, my eyes get lost. I even have to look carefully to find the "X" to close the pop-up:
The reason for this phenomenon is that in dark mode the pop-up doesn't have a border, while in light mode it does. You need to add a border that contrasts with the background, just like you have one in light mode.
Then there's a similar issue when editing an event. Here's the light-mode version of that screen:
In dark mode, it's a disgrace. You can't tell where the fields are at all:
One's brain cannot tell instantly that what one's looking at is a form, and even though the user's conscious of this, a lot of user interface design deals, indeed, with sub-conscious clues. So, in essence, having to THINK about this creates a delay. It should be evident that certain information on the screen is a user-editable field. In light mode you have the gray background. In dark mode there's nothing, so the user gets lost. This is a BASIC UI design principle.
Then there's the fact that keyboard shortcuts seem to be gone. With the old interface I could press ctrl-enter and save an event. Or esc and discard my changes.
And another thing that's gone is the ability to tell, AT A GLANCE, whether a given day had events by looking at the small full-month calendar on the bottom-left corner of the screen. That was an extremely useful feature.
I honestly don't know what you were thinking when you forced this useless change down your users' throat. Your UI designers are obviously not the most competent when it comes to matters of perception. Aesthetically-wise, well, they're definitely following whatever design fad is in effect.
The bottom line is that this new UI design adds absolutely no value to the zoho calendar experience (in fact, it subtracts).