For GTK theme configuration, I installed the gtk-chtheme package that allows you to change which GTK 2.0 theme to use, but this is something that can also be done from System Settings/Application Appearance/GTK+ Appearance. One nice thing about GTK 2.0 Change Theme is it shows you examples of what the different themes look like (check boxes, radio buttons, buttons, sliders, scroll bars, font, etc.), which GTK+ Appearance does not show you.
Out of the box, only two GTK themes are installed, Raleigh and another one I don't remember - both looked ugly to me. At some point I got additional themes installed (Clearlooks, Crux, Industrial, Mist, QtCurve, Redmond, Thinice and oxygen-gtk). I'm not exactly which packages got me these themes, but the different packages I have installed are gtk2-engines-qtcurve, gtk2-engines, libcanberra-gtk3-0, libcanberra-gtk3-module, gtk3-engines-oxygen, libcanberra-gtk0, and libcanberra-gtk-module. There was also libwxgtk2.8-0 that was needed for p7zip. The canberra packages were something that seamonkey (browser) needed to look right. (Note that I am running KDE 4.8.2 on Mint 12 KDE.)
I currently have my GTK theme set to QtCurve (though my main KDE theme is Plastique, something I grew fond of back with KDE 3). Unlike Plastique, QtCurve is very configurable (very, like insane - every aspect of the theme). QtCurve can probably be set up to look like Plastique but I haven't spent the time.
Anyway, I learned that when the GTK theme is set to QtCurve, it can be configured (fine tuned) using the Configure... button on System Settings/Application Appearance/Style when the Widget style is set to QtCurve. These changes then apply to the GTK QtCurve theme (though the GTK programs may have to be restarted to see the changes). But because of some kind of bug maybe, changes don't show up immediately unless I select another Widget style, click Apply, select QtCurve again, and finally click Apply again.
KDE Neon (formerly Mint 17.3 KDE): AMD FX-6300, 16GB RAM, GTX 660Ti (2GB), Gigabyte 990FXA-UD5
(with Windows XP, Windows 10, and Mint 18.3 KDE VMs on VirtualBox)