How can I theme the toolbar icons?

ROX-Filer uses the standard Gtk stock icons system to change the toolbar icons. Put something like this
in your ~/.gtkrc-2.0 file:

pixmap_path "/path/to/icons"
style "normal" {
  stock["gtk-close"] = {{"close-icon.png"}}
  stock["gtk-go-up"] = {{"up-icon.png"}}
  stock["gtk-home"] = {{"home-icon.png"}}
  stock["gtk-refresh"] = {{"refresh-icon.png"}}
  stock["gtk-zoom-in"] = {{"zoom-in-icon.png"}}
  stock["gtk-zoom-fit"] = {{"zoom-fit-icon.png"}}
  stock["gtk-jump-to"] = {{"jump-to-icon.png"}}
  stock["gtk-sort-ascending"] = {{"sort-icon.png"}}
  stock["gtk-help"] = {{"help-icon.png"}}
  stock["rox-show-hidden"] = {{"hidden-icon.png"}}
  stock["rox-show-details"] = {{"details-icon.png"}}
  stock["rox-select"] = {{"select-icon.png"}}
widget "*" style "normal"

See the list of stock icons in the Gtk documentation for details.