GTK 2.x
Sample settings file (.gtkrc-2.0)
gtk-theme-name="Mire v2_Lime"
gtk-icon-theme-name="hicolor"
gtk-font-name="Verdana 10"
gtk-toolbar-style=GTK_TOOLBAR_ICON
Qt5 style
Install qt5-gtk2-platformtheme package and add to .profile
export QT_QPA_PLATFORMTHEME=gtk2
Fix font anti-aliasing (green border around text) in GIMP 2.8+
Add /etc/gimp/2.0/fonts.conf
<fontconfig>
<match target="font">
<edit name="rgba" mode="assign">
<const>none</const>
</edit>
</match>
</fontconfig>