based on its behavior, gthumb appears to not have styled the headerbar with '.titlebar'

Themes which style the min/max/close buttons on gthumb's headrebar (e.g. 'Xfce Evolution 5.0.0', d/l here https://sourceforge.net/projects/xfce-evolution/files/xfce-evolution-5.0.0.zip/download) do not style those buttons in gthumb 3.6.1
However, if i change the min/max/close buttons' gtk3 css selectors from 'headerbar.titlebar' to 'headerbar' (i did this in 'Xfce Evolution 5.6.0+', d/l here https://sourceforge.net/projects/xfce-evolution/) then the buttons do get styled.