In Thunderbird 115 on Windows 10 x64 I have a problem with the following theme:
Its the event remainder dialogs (remainder and remainder->details): If I resize them vertically, a black bar appears at the bottom and the buttons vanish behind it.
The developer has no public contact information, that does make it difficult. Could you file a bug in the Thunderbird:Theme bugzila component, so the Thunderbird Team can check if there is a general Thunderbird bug causing this behavior?