Sad to see Zotero following the "modern" trend of making it impossible to tell which window is active. How can I restore the standard title bar and menu bar?
You're conflating things here. Whether or not there's a dedicated title bar doesn't have much to do with being able to tell if a window is active. E.g., Zotero on macOS uses pronounced fading for inactive windows, in line with system conventions:
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https://s3.amazonaws.com/zotero.org/images/forums/u6/j7a4mlvjdamw4d6bpl3h.png
https://s3.amazonaws.com/zotero.org/images/forums/u6/ngsmny7rryfiyppj123k.png
On Windows, some things are faded (window controls, text-field underline, row selection), but not as many, and we could probably do more.
In any case, if you're having trouble with something, provide some screenshots with examples. As it is, we don't even know what OS you're running.