If system-software-install-symbolic.svg is present in /Adwaita/symbolic/legacy/ folder, 
it will have the priority over the file placed in the /Adwaita-Red/16x16/legacy/ folder. 
As a result, PCManFM will not use 
/Adwaita-Red/16x16/legacy/system-software-install-symbolic.svg icon.

Adwaita/symbolic/legacy/system-software-install-symbolic.svg