This is a recorded message, please always provide the version of Geany, Glib and GTK (see lines near the top of Help->Debug Messages) and the operating system and version you are using.
And in this case, the desktop (don't assume everybody knows when Ubuntu switched from mir to gnome).
And did you try changing the theme as posted by the OP?
Needless to say WFM on Linux Mint Cinnamon (based onUbuntu 18.04) with Geany 1.37 (git >= 82590af4), en_AU.UTF-8 GTK 3.22.30, GLib 2.56.4