Was it an add-on theme, or is this the change in the "Default" (now "System" theme)?

If it was an add-on theme:

Please check the Add-ons page (Ctrl+Shift+A), Themes list. Can you re-enable your preferred theme there?

If it was the old "Default" theme:

This add-on theme has the dark navy blue top bar, if you're missing that:

https://addons.mozilla.org/firefox/addon/photon-colors/

https://addons.mozilla.org/firefox/addon/photon-colors/ No, this is a dark Navy Blue (virtually Black) not Windows System Blue, as the previous FF versions were. Please revert this default theme to the Blue we had before, or at least have it as an option.

Hi herodotus2007, on my Windows 10, it was very dark. Example screenshot attached. Are you also on Windows 10?

Anyway, the problem is that there are a zillion themes and there isn't a way to search by the shade of blue. You might need to create your own theme using this extension:

https://addons.mozilla.org/firefox/addon/firefox-color/

The very last thing I wanted to do today is page through thousands of your UGLY, ILLEGIBLE, horrible themes to find one that I can use. Why in the heck did you guys remove the old themes? And why, oh why, don't you at least allow a search to be done, i.e., dark with white letters, black letters, colors, plain, nudies -- I don't want to see a bunch of nudes early in the morning. Does anyone at Firefox have the ability to think about the user end? I am SO angry.