I am on Linux, but have had the same, or similar, problem.
I tried a couple of generally recommended plugins, but could not get them to work.
Running Zotero 6.0.26 currently, I did the following instead:
[PLEASE PROCEED WITH GREAT CAUTION: I cannot be held responsible for others' mistakes]
* Open 'Zotero Preferences', and click the 'Advanced' tab.
* Click on the 'Config Editor' button.
* If you wish to proceed, agree to "I accept the risk!"
* A list of settings fields will appear.
* At the top of the pane is a search field:
* Type "dark" (without the quotes) and hit the tab key on your keyboard.
* You should see row entries as follows:
* reader.color_scheme.values
* widget.chrome.allow-gtk-dark-theme
* widget.content.allow-gtk-dark-theme

There are four columns, with a "Value" column at the far right. If you double click the "Value" entry for each of the following rows:

* widget.chrome.allow-gtk-dark-theme
* widget.content.allow-gtk-dark-theme

... if the entry read "false" for either or both, it should now have switched to true. Do this for both entries if you want content and chrome features to switch to use the dark theme.

You should now carefully close the 'about:config' window.

Restart Zotero.

You should now be soaking up a new look Zotero, replete with all dark goodness and calm.

Best Wishes,

XinoG