It works that way in TB 59beta, but it probably won't arrive in the release version until the next release, TB 59 or 60.

https://bugzilla.mozilla.org/show_bug.cgi?id=1378320

In the meantime, you might be able to accomplish something similar a with userChrome.css file:

https://support.mozilla.org/en-US/questions/1094160

The info that a fix is coming is interesting, thank you, but the other link to the firefox workaround doesn't fix T'bird. Actually, my firefox displays both active and inactive titlebar colours correctly without any colour-specific modification. Irritatingly the title bars on dialogs created by T'bird are coloured correctly.

I realize the link refers to Firefox, but userChrome methods apply to TB, too.

The current Firefox, 58, has theme modifications that weren't made in time for TB 52; some of them are built into TB 59b, as noted.

It is odd that the titlebars for Address Book, Options and Write in TB 52 do conform to W10 colours, but the main window does not.

OK, looks like I will just have to possess my soul with patience and wait for TB59. Thanks for the info/help.

To misquote the famous Windows haiku,

Yesterday it did not work. Today it does work. Windows is like that.

When I started Thunderbird today I noticed that I had a properly coloured title bar. I had been using a T'bird theme which I deleted as it had effects I did not like so I reinstalled it and lost the titlebar colour again. It appears that using themes is incompatible with the workaround. Hopefully it will not be incompatible with the final fix.

Thanks for your help.