Comment 4 for bug 1655647

If you compare the screenshots by overlaying them in gimp and using difference mode for the top layer you can see that the icons are actually the same size, the problem is about margins and paddings in the buttons.

A change like this in the gtk style improves the situation a little bit:

#ToolToolbar button,
#SnapToolbar button {
  padding: 1px;
  margin: 1px;
}

but this is just for example.

Another related problem is that the scollbars are very thick depending on the
theme, see the attached screenshot made with the default Adwaita theme on linux (Gtk 3.22).

A change like the following, suggested by su_v improves the situation a little bit:

#LockGuides,
#StickyZoom,
#CMS_Adjust {
  padding: 0px;
}

From https://gitlab.com/su-v/inkscape-gtk3-theme-small/blob/master/ui-3.20/style.css#L310-315

So, some issues are just styling problems and can be debugged by playing with CSS in the gtk inspector:
GTK_DEBUG=interactive ./bin/inkscape

A possible way to find a theme which matches the 0.92 as much as possible would be to open a stable copy of inkscape and then open a development copy with the inspector on top of it and set the window partially transparent and then try to come up with a theme as much similar as possible to the gtk2 version of inkscape.

The window can be made transparent in the inspector with a CSS snippet like this:
window {
 opacity: 0.95
}

However not all issue are fixable in the theme, for example to rescale the corner images in the canvas we can set min-width and min-height from CSS, but not the pixel-size property.

I don't think I'll have the time to work on a patch myself but I wanted at least to provide some comment.

Thanks,
   Antonio