This Geany plugin requests a dark theme variant to be used for Geany's UI. It requires the use of a GTK3 Geany build, and a GTK3 theme that has a dark variant (like the default Adwaita theme and other popular themes).
Fetch geany-plugin.mk from https://github.com/b4n/geany-plugin.mk inside the plugin's source directory:
$ git submodule update --init
And then run make and make install (that last one possibly as root).
Enable the plugin in Geany's Plugin Manager (Tools -> Plugin Manager, and select the Dark UI Theme plugin).