The larger check works well in the headerbar, but not inline in various UI elements. This reverts the larger check since the latter is more common. For selection mode, a separate larger icon (selection-mode-symbolic) has been added to adwaita-icon-theme.
These were showing up as missing icons when opening
the Inspector with the hicolor icontheme:
system-search-symbolic
go-previous-symbolic
go-next-symbolic
display-brightness-symbolic
This subdirectory gets in the way when integrating the inspector
build more fully with GTK+, and does not really add anything.
Just move everything one level up.