Oh my giddy aunt - you've done it!!
That's it! Worked for me and - as soon as I read 'Match Theme Colour' in your post I knew that would be it! That's the one thing I remember changing after reinstalling ID - thus repeating the problem.
Don't get me started on Sync Settings though - I've NEVER got that to work properly since I first saw it. It seems to work backwards whereby I mean - you'd think you'd have 'master settings' on the cloud and then every time you hit sync settings button bottom left, it would changed to you preferred 'saved' settings - but it doesn't - instead, every time you hit that button it makes your cloud version the same as the document you have open. And only when you open an unsynced copy of ID does it ask you if you want to sync - and last time I clicked ok (or yes) it did... nothing. Which is why I had to manually change all my settings again (including the match to theme colour) ...what did I say about not getting me started...?
Thanks J van E - yes it does sound like a bug - I'd like to see it fixed - I quite like having a dark preview background...
@monica singh