Comment 2 for bug 1764021

This is basically a GTK+ issue - ironically just closed today due to their GitLab migration:
  https://bugzilla.gnome.org/show_bug.cgi?id=710650
I'm not sure if they still accept GTK+2 bugs in GitLab (development is focused on GTK+3 and above these days) but you might try your luck.

That said I have to be honest though: Windows-related issues were never a high priority and Windows 7 claasic theme is not widely used either, so your best bet is probably finding/motivating somebody who knows their way around GTK+-theming and can create a patch. (I'm not that person but I'd be happy to make sure it's included in Inkscape in some way).