@Johndeep You have several options to change the theme used by Geany through the regular GTK means. E.g. one for GTK3 is to set the `GTK_THEME` environment variable, like `GTK_THEME='Adwaita:dark' geany`, which will use the dark variant of the *Adwaita* theme for launching Geany. You can also play with the setting I mentioned previously, or things like `GTK2_RC_FILES` for GTK2, etc.
I really don't think Geany should try and not follow the theme by itself; but I guess a plugin could do that. I just wrote such a plugin simply requesting the dark theme variant for the fun of it and to prove my point: https://github.com/b4n/geany-plugin-dark-theme