All icons used in Inkscape should now be themable. You can find out the names by looking at the IDs in icons.svg, at icon names in in share/icons/hicolor/scalable/*, or at the file src/ui/icon-names.h.
All names are defined in a new file, ui/icon-names.h. This file should be used for all icon names used in Inkscape. I also removed stale files which attempted to do the same but weren't used universally (ui/stock.h, ui/stock-items.h, inkscape-stock.h).
Right now Inkscape uses the discrete SVG icons. I timed "hot" startup (using "time inkscape --verb FileQuit" twice and noting the second run) and it seems that it's actually faster (9.281s) than using icons.svg (9.799s), probably because the RSVG renderer which renders the SVG icons in Gnome is faster than ours.
Regards, Krzysztof Kosiński