When a theatre is 'dark' it means that it is closed. This may be temporary, e.g. between shows, or it may be semi-permanent, e.g. when it is closed for refurbishment (sometimes it is uncertain when this happens whether it will actually open again) and sometimes it means that the theatre is completely out to action, lost or gone!
UKTW Historical ArchiveDark archive
with 105 listings to Sunday, 27th January 2019.