To edit the categories in the thunderbird lightning calendar could go to Tools -> Options -> Lightning -> Categories, but in the current version (60.3) this doesn't seem to be possible anymore as Tools -> Options was removed. I also couldn't find it under the Preferences.
How is it currently possible to edit the calendar categories (delete, change color etc)?
In more recent versions of Thunderbird, the settings for the calendar categories can be found under Edit- Preferences, Calendar tab, Categories. The settings may also be reached from Tools - Add-on preferences - Lightning.
(with thanks to flawr for the screenshot!)