Agree with @kugel- regarding a "reasonable" default theme on Windows and Mac, but how to choose. On Mac I think its @techee`s choice since he is the one who will do it. Therefore on Windows its @eht16's choice, although that will probably result in a WIndows 7 theme, even though the indication is many Windows users are using Windows 10 ;-P
For Linux we should simply leave it alone, Geany properly follows the selected GTK desktop theme automatically, which is the theme selected by the desktop menus/prefs app/look like me button/whatever the desktop provides. We should not attempt to provide anything, it will almost certainly be overridden or conflict.
And on all platforms `geany.css` is there for tweaks of things the theme doesn't define or defines badly.