This problem affecting the main window and preview window is also present in all other themes (e.g. Ambiance, Greybird, Arc) that I've checked. Although the lack of slider borders is less critical, it is somewhat difficult to accurately select a custom area to scan with the help of the mouse pointer only.
Probably it's the fault of that specific app (selection area borders in many other apps are quite visible although I cannot be sure which is GTK2, which is GTK3 ) but I hope some fix is not too difficult to implement in the theme.
This problem affecting the main window and preview window is also present in all other themes (e.g. Ambiance, Greybird, Arc) that I've checked. Although the lack of slider borders is less critical, it is somewhat difficult to accurately select a custom area to scan with the help of the mouse pointer only. Probably it's the fault of that specific app (selection area borders in many other apps are quite visible although I cannot be sure which is GTK2, which is GTK3 ) but I hope some fix is not too difficult to implement in the theme.