Outlook Calendar Appearance

I'm afraid this isn't it; I've been using the Dark Grey and Calligraphy themes for many years, and have tried amending all of these settings. The appearance changed suddenly yesterday. Even the category colours have changed, for example the blue (used for most of my appointments) was a darker blue, and the pink you can see in the image was much paler. 

I can't see any updates in my Windows folder, but am suspecting that something has been updated and this caused these changes.

Any other ideas?

I agree. my partner's screen has done exactly the same thing and its ugly in the extreme. We need a way to change it back.

I've made a request on uservoice, pending moderator approval currently:

https://outlook.uservoice.com/forums/322590-outlook-for-windows-desktop-application/suggestions/40557520-option-to-remove-gridlines-in-dark-grey-theme

Partly trough.
Colour scheme:

Dark grey scheme; lines used to be gray (like in colour scheme) instead of "hard black"

I have two computers, both Win10. One is showing this issue and the other is not. The version of both is up-to-date and the same but only one of them shows this problem. The one that does NOT show this problem is my work provided computer and has some domain management controls in place though I believe those managing O365 are minimal.

The version of Office/Outlook on both computers is "Microsoft 365 for Enterprise" - Outlook version 2005 (Build 12827.20268 Click to run) - Current Channel

That seems to suggest it is not a version issue. Thought it is possible that the "What's new" option was selected on the problem machine (a few weeks back) and not selected on the good machine. I cannot see the switch to show "whats new" in Outlook on either machine.

I had two that were apparently the same and as far as I can remember both had the same version numbers! One had the black lines and one had not changed. Then there was an update on the one that remained the same and lo and behold it too changed. Still no apparent change in numbers!!! Perhaps I missed a minor number change, (that would be what I'd suspect if someone said this to me), but I don't think I did.

The "what's coming" item seems to be missing on mine now - twas a good idea though.

Might be interesting to keep a close eye on the one that is unchanged and see if/when it changes and document exactly what happens to bring about the change.

EDIT: If you click on Help >> What's New  It will tell you that We don't have anything for you right now. So that's why the button has disappeared.