TB 68+ no longer supports complete themes that update all the UI colors. Fortunately, the functionality can still be duplicated by extensions. So far, I've only found a couple, and both of them are dark. Still, either one is an improvement on TB's native themes. And you can always tweak them if you're comfortable unzipping the XPI and editing the CSS.After Thunderbird has been relaunched, Lightning will be back and you will have access to your calendar data.
- Go to the Advanced section of Thunderbird Preferences and click on Config Editor...
- Copy or start typing extensions.installedDistroAddon.{e2fda1a4-762b-4020-b5ad-a41df1933103} into the search box
- When extensions.installedDistroAddon... is listed in the preferences list, right-click on it and choose Reset from the context menu - the preference now isn't bold anymore and its value is empty
- Close the editor, leave the Thunderbird options, open Thunderbird's add-ons manager and switch to the list of installed extensions
- If Lightning is listed there, click on Remove to the right of the list item
- Restart Thunderbird
If you're interested, the two extensions are Monterail Full Dark 2 for TB>68 and DeepDark for Thunderbird.