I did contact the Plata dev, and he explained why it wasn't going to do what I wanted...
But all I had to do was change the icon theme to a "dark" variant, and the icons are now nice and white. So, Zim looks great in a dark theme.
Note to self: remember change icon theme to "dark" when using a dark theme.
Btw, it was the default on the install to have Plata-Noir and Papirus. Any reason it is not Plata-Noir and Papirus-Dark?