Unfortunately Roland is right, you are probably best doing this manually at the moment.
However, it is possible to change Wine's system-wide default registry settings by editing /usr/share/wine.inf -- this is the registry that new Wine Prefixes get created with. I'm not sure how to add a theme in there, although you could look at the registry of a current Wine install after you change the theme for hints.
You can enable 'Global Dark Theme' mode on Unity 7, the same way it's enabled on Gnome Shell. Just install GNOME Tweak Tool by clicking here:
Or run this in Terminal:
sudo apt install gnome-tweak-tool
When it's installed, open Tweak Tool from the Dash and toggle on 'Global Dark Theme' switch as shown below:
From now on you can use the dark mode of themes under Unity 7 the same way you use them on GNOME!? If it's not working, just check if the theme implements GNOME's dark theme mode.
GNOME Tweak Tool and most of the options it provides are tailored for Gnome Shell and so may not work as intended on Unity. Coz of this, many users prefer not to install GNOME Tweak Tool and choose Unity Tweak Tool over it. If for any reason you don't wanna install GNOME Tweak Tool, this is how you can enable the 'Global Dark Theme' mode manually:
Step 1: Open the GTK3 configuration file in Text Editor:
Step 2: Paste this into the file opened by Text Editor: