For GNOME users, one can now match the system accent color with Gtk3 applications with adw-gtk-theme >= 5.5-2.
I’ve now added accent-color-change to the repos. It’s a script to change the accent color for Gtk3 and Gtk4 applications that do not use Libadwaita created by the author of adw-gtk3 (adw-gtk-theme in the repos).
EDIT: Updated with new script.