This is a known issue.
This theme doesn't support Ubuntu 14.04, because its GTK version (3.10) is too old. I'm working on GTK 3.14 support, which will make the theme compatible with Ubuntu 15.04.
I'm sorry about this, but I don't have enough resources to backport the theme to older GTK versions.
Depending on which Arc theme you are using, edit the corresponding
.css file. Even better, edit all of them and fix all Arc themes on 14.04:
sudo gedit /usr/share/themes/Arc/gtk-3.0/gtk.css
sudo gedit /usr/share/themes/Arc-Dark/gtk-3.0/gtk.css
sudo gedit /usr/share/themes/Arc-Darker/gtk-3.0/gtk.css
Once gedit opens press Ctrl + F and type the word
Now simply replace the highlighted word
dashed with the word
i.e. change this line:
outline-style: dashed; to this:
Save, close, and you're done. Enjoy.