When resolving an icon name, we currently look in all themes in
order whether one of the requested names matches.
That means, a "keyboard" icon from the configured theme will
trump over any of "keyboard-shift", "keyboard-brightness" and
"keyboard-preferences" from lower-ranked themes (like "Adwaita"
and "hicolor").
That applies to custom icons we include in the resource, which
are added to the fallback ("hicolor") theme.
We are less bound by the named-icons spec than GTK, so we can
adapt the lookup to prefer the most accurate match to matches
from the highest priority theme.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2824>
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