How can i find, which themes are installed
GTK+ has preset paths to look in, usually `$PREFIX/share/themes` and somewhere below your home directory (depends on the platform, on win32 it's a funny path like AppData or such, depending on Windows version). There is probably also an environment variable to control where GTK+ looks.
respectively where does Geany look for them?
Geany itself isn't involved, but the GTK+ bundled with Geany will look below Geany's installation directory as that's its `$PREFIX`, in addition to the themes directory below your home directory.
I thought that, if the GTK theme chooser is offering me a theme for selection, that the theme is there.
AFAIK there is no official GTK+ theme chooser, which if true means you may have installed some 3rd party utility (possibly bundled with a 3rd party release of GTK+?).
C:\p\GTK\2.0\share\themes\Nodoka-Squared
This `C:\p\GTK\2.0` directory doesn't come from Geany, it must be a separate GTK+ distribution that you installed separately, so the GTK+ bundled with Geany won't look there for themes. The GTK+ that comes with Geany would be found under it's directory in `C:\Program Files (x86)\Geany`, and most likely looks for themes in `C:\Program Files (x86)\Geany\share\themes` (can't say for sure as I don't have a win32 installation available).