The UI elements (apart from the editing area) are using the configured GTK look and feel, so changing that will change the ones in Geany. However, if you're using GTK3, there is a setting that asks for a dark variant of the theme, and there is a Geany plugin that enables the setting for Geany if you wanna try it out: https://github.com/b4n/geany-plugin-dark-theme

First check if your Geany is GTK2 or GTK3 `Menu->Help->Debug Messages` near the top says GTK version, since the plugin only works for GTK3 IIUC.

Thanks everybody I have find how to change the global theme on my rpi ! Problem solved !

Reopened #1935.

Afraid you can't use @b4n's plugin, no.

But you can use the standard GTK2 themeing which isn't really a part of Geany, but is documented as part of GTK. Note that the simplest is of course if you select a dark theme for your desktop Geany should follow (assuming the person who wrote the theme did a good job).