I use Zim quite a bit, but there are a few other apps that caused me to change all my GTK apps to a dark theme, using the Gnome Tweaks application.
To try this method, first install the Gnome Tweaks tool:
sudo apt install gnome-tweak-tool
Then, hit the Super key or click the "Show Applications" button on the bottom left of the desktop, and type "tweaks".
After tweaks comes up, select the "Appearance" tab. You'll notice that the first section is "themes". For Zim, the "Applications" setting is the one controlling the theme.
On 18.10, I was able to select Adwaita-dark, and Zim picked up the settings immediately.
There's a relevant makeuseof.com article with a lot more information on how to change the gtk theme, if you need it.
This is a quick and easy change, so you can change back if the aesthetic doesn't please you.