For those of us with OCD:

When you add Proton Calendar (calendar.proton.me) as a PWA to your macOS device (e.g. using Brave browser), you get a white square icon that does not match established macOS style guides (no rounded corners). Current fix is to override the icon with a custom one via the Finder "Get Info" option. This is probably an issue for other Proton apps, but I have not checked those.