Qt Oxygen for GTK Apps
This process allows you to have the Qt Oxygen theme or another theme of your liking, applied to all GTK applications. All of the required packages can be obtained from the AUR.
Install oxygen-gtk2, you may also wish to install oxygen-gtk3 for GTK 3 applications. I currently do not have any GTK 3 applications installed, so I neglected this step.
yaourt -S oxygen-gtk2
Next, install kde-gtk-config in order to apply your theme.
yaourt -S kde-gtk-config
To configure your theme, go to System Settings > Application Appearance > GTK Configuration. From here, you can select your desired GTK2 and/or GTK3 themes.