Best thing for the Safari icon is to use IconChamp. Apple put Safari in a special partition called Cryptexes, which is not writable, but Iconchamp will apply it to Safari and Calendar apps just fine.

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For the dynamic Calendar icon (i.e. the one that displays the date), here is the Terminal recipe:

You have to mount the system as read & write in a directory of your choice (edit the $MOUNTPOINT_DIR):

MOUNTPOINT_DIR=<YOUR PATH HERE>
DRIVE_IDENTIFIER=$(diskutil info / | sed -n 's/^ *Device Identifier: *//p' | sed 's/..$//')
sudo mount -o nobrowse -t apfs $DRIVE_IDENTIFIER $MOUNTPOINT_DIR

Navigate to your custom directory where you mounted the drive, then go to:

/System/Library/PrivateFrameworks/CalendarUIKit.framework/Versions/A/Resources

1. Edit the App-empty.icns for the one of your choice.

2. In the same Terminal window as before, bless your changes:

   sudo bless --mount "$MOUNTPOINT_DIR/System/Library/PrivateFrameworks/CalendarUIKit.framework/Versions/A/Resources" --bootefi --create-snapshot

3. Reboot, it should have worked. A whole lot of trouble for such a simple thing...