All,
I have loaded Inkscape 1.0 both on windows and on Linux (Arch). I have both on my laptop. The biggest problem is that toolbars that used to fit properly now spill-over into elipsized menus and the spacing between the rulers and icons are now double what it was before. In 92.5 the interface showed 24 icons on the vertical toolbars. In 1.0 only 17 are visible. (despite all icons sizes being set to "smaller" in preferences)
On Arch I have the Adwaita icon them (about all that is available anymore). On Windows I guess it comes with the icons. Both interfaces are almost identical in the size growth of the icon size and the ruler-to-iconbar spacing.
This isn't a problem unique to Inkscape. All apps making the Gtk+2 to Gtk+3 jump have faced the issue. Gimp, Mozilla, Geany, etc.. and most have found a way to solve the problem. I know it is frustrating for you as well, but for small screens like laptops -- it is a big issue.
css doesn't provide a solution a global solution, despite the Gtk+3 documentation pointing in that direction with https://developer.gnome.org/gtk3/stable/GtkToolbar.html
Searching, there are old solutions suggesting entries in ~/.config/gtk-3.0/setting.ini similar to:
gtk-icon-sizes = panel-menu=16,16:panel=16,16:gtk-menu=16,16:gtk-large-toolbar=16,16:gtk-small-toolbar=16,16:gtk-button=16,16
Gimp 2.10 has somehow solved this issue so that interface is comparable to Gtk+2 and will still allow pickup of the Gtk+2 themes. I'm not sure how they did it. Whether they wrote a partial theme to handle it or what.
I'll keep searching and I'll keep working with the interface to see if I can find something that can work. The actual solution needs to come from the gnome developers of Gtk+3 providing programatic control of the toolbar icon sizes rather than leaving them to be overridden by the themes.
The fact that Gtk+3 was such a moving target for so long and the fact toolbar icon size has gone unaddressed this long is a shame. You all have put so much good work into Inkscape for the 1.0 release, it must be really frustrating that the new Gtk+ version doesn't provide near the polished look that Gtk+2 provided off the shelf.