The only things I could find are relating to 2020 and before, so it may/may not work.
If you look here, it explains about the config editor:
This article describes how to configure the Lightning add-on for Thunderbird.
In there, can you see a row called:
If so, is it set to false? If it is, see if it works by changing it to True.
If that doesn't work, there is an add-on here, but apparently according to the comments, it may not work for the latest versions of TB:
Allows several tweaks to the Thunderbird/Lightning calendar user interface including new selection styles and repeating event icons. Mimics the colors and styles found in MS Outlook.