Useful program to preview themes is
lxappearance from LXDE project. It makes it handy to preview and edit GTK themes, icon themes, X mouse cursors and default font settings. It is free software (developed under GNU GPL). It can be installed from official Arch repositories
$ pacman -S lxappearance.
My favourite GTK theme is Boje by nale12 (Turquoise version). It is free software (developed under GNU GPL).
gtk-theme-boje package is available in AUR.
I liked the look of Flattr icon theme for a while and fortunately, author have recently liberated the license, (changing it from CC BY-SA-NC 4.0 to CC BY-SA 4.0) making it free software!
flattr-icon-theme-git is available in AUR.