This question already has an answer here:
When i'm switching the theme in Gnome Tweak tool some applications like calculator and system monitor doesn't switch it. They are just stuck on Adwaita (default) theme. In another apps like Files everything is good. Is this a bug of a new version of Ubuntu or i am doing something wrong? I am also using Ubuntu 16.04 and all these apps are switching themes perfectly.
Current theme is Arc-Dark but some apps are still use Adwaita: