Upstream our window list theme changes

Any non-upstreamed change we apply is essentially a bug (due to its maintenance cost), so this ticket is about tracking the upstreaming process of the changes applied for #12233 (closed).

Upstream bug report: https://gitlab.gnome.org/GNOME/gnome-shell-extensions/issues/22
Upstream merge request: https://gitlab.gnome.org/GNOME/gnome-shell-extensions/merge_requests/3

Related issues

Related to #12233 (closed)
Blocks #16209

Original created by @intrigeri on 12415 (Redmine)

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