Since Inkscape is finally moved to GTK3, it's time to start GTK2
deprecation process.
What this does is just a hiding direct interface for setting GTK2 themes.
Despite of that, GTK2 will be set implicitly along with the GTK3 theme.
Some FAQ:
Q: What if there is no GTK2 theme with the name of GTK2 one?
A: Chances of that is close to zero. Most, if not all, GTK3 themes
contain gtk-2.0 folder along with gtk-3.0 one. Even if there is such
a theme, currently it also will not work. That's because of xsettingsd,
that overrides GTK2 configuration file.
Q: What about Breeze Dark GTK2 theme for GIMP?
A: GIMP has a dark theme inside. Breeze-dark will not work for the same
reason as the above - xsettingsd with only one theme for GTK2 and GTK3.