PS if you are on Windows there is a theme provided because so many people hated the GTK default dead tortoise (Adwaita, google it) theme and Windows users couldn't be bothered to download alternate GTK themes, but the argument and disagreement that went into getting to that makes the chances that anyone will provide a dark version fairly low without an actual pull request and sufficient agreement.

Go to your gnome instalation location `~\geany\share\themes\Prof-Gnome\gtk-3.0` and change name of `gtk` file to 'gtk-dark'.