Use standardized icons
For example, preferences-system-search
from https://gitlab.gnome.org/GNOME/geary/-/blob/main/ui/components-headerbar-conversation-list.ui#L57 is not in https://specifications.freedesktop.org/icon-naming-spec/latest/ar01s04.html. Therefore, it's not themed correctly, e.g. in KDE. system-search
could be an alternative.
I understand that not all required icons might be in the standard set but the amount of those should be minimized as much as possible.
Edited by plata