
you can see the topbar is same with theme ambiance in ubuntu 17.10
Ubuntu 17.10 uses a custom version of GNOME. Not a big fan personally, I learned to love the GNOME experience. That's why I'll try to get back to the vanilla GNOME (and there I can use Arc): http://www.omgubuntu.co.uk/2017/10/install-vanilla-gnome-shell-ubuntu-17-10
Just understood your issue while discussing with a friend, that's not a bug, you have to enable Arc for the shell, too.

See this how-to for instructions: https://www.howtoforge.com/tutorial/how-to-theme-gnome-shell/
You may need to install the User Themes extension from https://extensions.gnome.org/extension/19/user-themes/
yes. you solved my problem.
@phpbugcn Could you close this issue then?
Just for better issue tracking: The new gnome shell then looks like this:

Most helpful comment
Just understood your issue while discussing with a friend, that's not a bug, you have to enable Arc for the shell, too.
See this how-to for instructions: https://www.howtoforge.com/tutorial/how-to-theme-gnome-shell/
You may need to install the User Themes extension from https://extensions.gnome.org/extension/19/user-themes/