@MicroJoe all widgets except the editor widget already use the GTK theme settings. The editor widget does not use the GTK theme since the theme does not cover the complexity of mapping syntactic entities to styles. Thats what the Geany colour scheme (sometimes misnamed "theme" as well) does. But a colour scheme contains no information that its light or dark so even if Geany can determine from the GTK theme that it is dark, it can't select a colour scheme since they are not marked light or dark, and even if they were, which one is the best to select as a dark version of whatever the user has chosen as their light colour scheme.
IMHO it would be a waste of somebodys time making that work anyway, how often do you switch from dark to light? Just select the colour scheme from the menu, and if you don't have one, download one from https://www.geany.org/download/themes/ or make your own.
Also I note that browsers don't turn the window dark just because the desktop theme does dark.