1. Create folder "colorschemes"

C:\Program Files\Geany\colorschemes

2. Put this file, "steampunk.conf" 

https://gist.github.com/kimworks/58158d6e023bc22e8d27f1ec9a87e877

3. Go to menu ⇒ View ⇒ Change Color Scheme ⇒ Select Steampunk or other dark color scheme

4. Find this folder, or create one

C:\Users\<username>\AppData\Local\gtk-3.0

5. Put this file, "gtk.css"

https://gist.github.com/kimworks/ab7f2f25cfc9b0c9b843239c20e9c7b6

It is not the greatest dark theme, but it's dark. :-) Find better one and use that.