Well, I have found that regardless of what GTK theme I use in the main GTK installation, Inkscape uses its own, as dictated by the gtkrc file in the Inkscape install folder. Which one is used is under Inkscape's control. What bothered me most at first was that regardless of the theme a person uses for all other GTK programs, Inkscape did not respect them. It seems that (forgive me if I get this wrong) Inkscape under Windows is staticly linked with GTK, as no GTK installation is required to use Inkscape, and thus any custom themes are useless unless the are explictly installed to the Inkscape folder (under etc\gtk-2.0\gtkrc I believe). Also, Inkscape ships with the GTK-Wimp theme, but doesn't use it. I understand the "doing its own thing" idea, but right now Inkscape seems to do its own thing anyway.
Sorry if this comes across a bit strong.
Kyle